The verse is talking about the abheda (non-difference) that exists between yantra, mantra and devata.
There is a very similar verse in the Kularnava Tantra's 6th chapter (86th verse).
yantram mantramayam proktam devata mantrarupini | yantre sa
pujita devi sahasaiva prasidati ||
Yantra is endowed with mantras (or mantric qualities); and devata is
of the ...
Sri Vidya mantras (particularly the Shodashi or Shodakshari mantra) are the most secretive among all mantras. So, the rule is to recite only after getting initiated from a Guru of that Parampara (of Sri Vidya).
Swami Shivananda, advises to recite a lot mantras in his book "Japa Yoga - A comprehensive treatise on Mantra Sastra ", but for Sri Vidya ...