Separately, what are yantras? In what kind of Hindu ritual are these used?
The above shows how Devi ChamundA appeared from the eye-brows of Devi amvikA. Now coming to your questions:
What exactly is this?
This is a framed yantra of Devi ChamundA engraved on a silver-plate with the famous navarNa Mantra written on it,
In Hinduism, Goddess Chamunda is an aspect of the Mother Goddess.She is closely associated with goddess Kali and is named Chamunda-Kali. Chamunda is one of the Saptamatrikas associated with Goddess Shakti along with Brahmani, Vaishnavi, Maheshwari, Kaumari, Varahi and Indrani. In Hindu scripture 'Sri Sri Chandi (Devi Mahatmya,) Maa Chamunda
emerged as Chandika from an eyebrow of aggrieved goddess Kaushiki or Anvika, a goddess emerging from "sheath" of Parvati (Sri Sri Candi, chapter 7, Mantra 5).
ChAmundA fought a fierce battle with the demons ChaNda and MuNda,
ultimately killing them. Goddess Chamunda-Kali took the slain heads of the two demons to goddess Amvika Kaushiki, Who became immensely pleased. Kaushiki blessed Chandika and bestowed upon her the title of “Chamunda", to commemorate the latter's victory over the demons.(Mantra 27)
According to a later episode of Devi Mahatmya,
Devi created Matrikas from Herself and with their help slaughtered the demo-army of Shumbha-Nisumha. Chmunda-Kali is here described as a Matrika who sucked all the blood of the demon Raktabija. Kali is given the epithet Chamunda in the text.Thus, the Devi Mahatmya identifies Chamunda with Kali. (Chapter 8, Mantra 54-57).
By the way, in Varaha Purana,
the story of Raktabija is retold, but here each of Matrikas appears from the body of another Matrika. Chamunda appears from the foot of the lion-headed goddess Narshmi. Here, Chamunda is considered a representation of the vice of tale-telling (pasunya). The Varaha Purana text clearly mentions two separate goddesses Chamunda and Kali, unlike Devi Mahatmya.
tells a different story of Chamunda's origins. She with other matrikas was created by Shiva to help him kill the demon Andhakasura, who has an ability - like Raktabija - to generate from his dripping blood. Chamunda with the other matrikas drinks the blood of the demon ultimately helping Shiva kill him. Ratnakara, in his text Haravijaya, also describes this feat of Chamunda, but solely credits Chamunda, not the other matrikas of sipping the blood of Andhaka. Having drunk the blood, Chamunda's complexion changed to blood-red. The text further says that Chamunda does a dance of destruction, playing a musical instrument whose shaft is Mount Meru, the spring is the cosmic snake Shesha and gourd is the crescent moon. She plays the instrument during the deluge that drowns the world.(Reference:https://www.devshoppe.com/products/goddess-chamunda-yantra-on-copper-plate#more-anchor)
The above Yantra contains the famous navarNa mantra of Devi : 'Om Aim Hrim Klim ChAmuNdai Vichche'. This mantra is known as navarNa mantra because this mantra contains nine (Nava) letters. We get the glory of this mantra in the Damara-Tantra as
O Destroyer of the darkness of ignorance, Ever-free, Supremest of the Supremes, The form of the AkhaNda-Brahmavidya, and the Embodiment of consciouness and bliss, we always meditate on you at our heart-lotuses. Your navarNa mantra has been explained variously. It is the giver of ever-Good. One can not apprehend even a fractional quality of this Mantra. Because of good deeds of many past lives, one can understand the glory of this mantra.
Om is pranava representing the Brahman. Aim is the vija that destroys the darkness of ignorance(Mahasarsvati), Hrimm is the vija of Sadrupatmika Mahalakshmi and Klim is the bija of Ananda-rupa Mahakali. The word 'Chamunda' means : chAnAm vuddhinAm vA sukhAnAm muNdam iva sthitA yA --She who is the pinnacle of the intellects and blisses. Vit+che = vichche. Vit means knowledge, cha is a neuter gender term representing the Truth and I is Supreme Bliss. The meaning of the navarNa mantra is : O SachchidAnandarupini Brahma-mahishi ChamuNdA, we meditate on you to acquire Brahma-vidya.
The rishis of this navarNa mantra are brahma-Vishnu-Shiva, chhanda is Gayatri,mUshnik and Anustup. Devis are Kali, Lakshmi and Sarasvati, Shakti- Nanda ShAkambhari and BhimA, vija is RaktadantikA, Durga and Bhramari, and the tattvas are agni, vayu and surya. (Reference: Sri Sri Chandi, Udbodhan, page 4-5).
The dhyAna-mantra of Devi ChAmunda is:
kAli karala-vadanA vinishkrAnta ashi-pAshini/vichitra-khatvanga-dharA naramAlA-vibhushanA//dvipi-charma-paridhAna sushka-mAnsAtibhairavA/ ati-vistAravadanA jihva-lalana-vishaNA//nimagnAraktanayanA nAdApurita-dingmukhA. (Sri Sri Chandi, chapter 7, Mantras 5-7).
meaning : the fierce form of [ChamuNdA]-Kali emerges with a frightening face. She holds sword, noose, and a strange-looking khatvanga.She wears the garland of human-heads. She wears a skin of tiger. She is just skin and bones, with a huge mouth, long and swinging tongue, and Her eyes are red and sunk.She fills the world with very loud sound.
What are yantras?
The word yantra is derived from the root 'yam', meaning to control. As worship of this helps to control all the internal and external enemies, it derives the name 'yantra'.
In what kind of Hindu ritual are these used?
Yantra is a part of the worship of the Tantrik deities.
According to Sritattva-chintAmani (17/2)
yamayati akhilam pApam trAyate mahato bhayAt/ sadhkam pujanad dhyAnAd tasmAd yantrah prakirtitah // meaning that a yantra when worshipped and meditated upon destroys all sins and removes all great fears and so liberates the aspirant.
According to Tantra, Yantra is a form of Devi. It is also known nas the dynamic graph of the Energies of Devi. It is a body of the Mantramayi DevatA.
yantram mantramayam proktA mantrAtmA devatA eva hi/ deha-Atmano yathA bhedo yantra-devatayos tathA.
Shakti-Sangama-Tantra (13/203) says
yantram tu griham iti uktam grihasthA devaytA matAh, meaning that the yantra is the house where the devatA resides.
Brihat-Tantra-SAra gives this yantra as the combination of one upward and one downward triangles with a downward triangle and bindu inside with encircled by an eight-petal lotus and sorrounded by bhupurs or gates on the four sides. The Yantra has to be written or engraved on vurga-patra, gold or siler plate:
sauvarne rAjate pAtre bhurje vA samyag Alikhet (Vrihat-Tantra-sAra, 10th ed, page 585).
To conclude, through Chamunda yantra one can get her blessings of the Mother Goddess. By keeping the Chamunda yantra, worshipping it properly chanting Her mantras , the aspirant invoke the Supreme Goddess and ask her to come into his life to fulfill all worldly and spiritual desires with ease.
The Chamunda yantra has the effect of bringing joy and confidence even while trying conditions are mitigated. Clears all demons away and keeps them away – the aspirant invokes the powerful Chamunda to protect and defend him from negative influences.
WARNING: SUCH TANTRIK YANTRAS/DEVIES SHOULD NOT BE WORSHIPPED UNLESS GUIDED BY AN ABLE GURU.