You're right, the term Adhibhuta is briefly described in the Bhagavad Gita, in Chapter 8 Verse 4:
O best of the embodied beings, the physical nature, which is constantly changing, is called adhibhuta [the material manifestation]. The universal form of the Lord, which includes all the demigods, like those of the sun and moon, is called adhidaiva. And I, the Supreme Lord, represented as the Supersoul in the heart of every embodied being, am called adhiyajna [the Lord of sacrifice].
So Adhibhuta consists of entities of the material universe, which are fundamentally transitory and impermanent.
For more elaboration we can turn to the commentaries on the Bhagavad Gita. Here is what Adi Shankaracharya says, for instance:
Adhibhutam, that which exists in the physical plane, i.e. that which exists by comprising all creatures;-what is it?-it consists of the ksarah bhavah, mutable entity. Ksarah is that which is mutable, which is destructible; bhavah means anything whatsoever that has orgination. This is meaning.
And here is what Ramanujacharya says:
By the word adhibhutam or those things pertaining to physical objects sought after by aisvaryarthis or fortune seekers are those things of a perishable nature including the supra subtle principles of sound, sight, smell etc. which are latent in their elements of ether, fire, earth etc and develop therefrom into their support system being the senses. All these things must certainly be comprehended and contemplated by them.
Madhvacharya's commentary on this verse is rather long, but it contains some useful quotes from other scriptures:
The Gita Kalpa states: "The Supreme Being dwelling within the physical body of all sentient beings is known as adhiyagna. Creation is a natural propensity of the Supreme Lord. It is the manifestation of His Divine will. Adhibhuta is the manifestation of temporary physical forms in the material existence. The jivas or embodied beings are a manifestation of adhyatma. The Supreme Lord's expansions such as Sankarsana or Hiranyagarbha are known as adhidaivam. The Supreme Lord Krishna is the Lord of all gods even Narayana the Lord of the spiritual worlds in Vaikuntha is an expansion of Lord Krishna." The Skanda Purana states: "That which is dwelling within exercising complete authority over the atma or soul is called adhyatmam. That which is external and separate from the physical body is called adhidaivam. Everything else which is different are the cause of gross, physical creatures therefore such activity is known as adhibutam." The Maha Kurma Purana states: "Adhyatma is that which being beneficial to the pure atma or soul extends unto the bodily limit. That which becomes useful to the jivas along with their physical bodies and other gross elements is adhibhutam. Beyond the scope of maya or illusory impressions superimposed upon the mind which is beneficial to divinity is adhidaivam."
To look at it from a more philosophical perspective, according to Samkhya, the Universe consists of Purushas, or conscious beings, and Prakriti, or unconscious matter. Prakriti by itself is formless, but in the presence of a conscious being it takes on forms. Initially it only takes on Sukshma or subtle forms, imperceptible to the senses, but then it takes on gross forms, i.e. the ordinary physical matter we perceive around us with our senses and which make up the physical bodies. It is the latter form of Prakriti, the substrate of the sensible domain, that is called Adhibhuta.
(On a side note, I recently wrote a dialogue exploring why the Samkhya school of philosophy ignores the existence of a supreme being, despite being founded by Kapila an incarnation of Vishnu. You can read it here.)