Mishnayot Menachot 2:5 introduces (as far as this tractate is concerned) the term "מתיר", which I take to mean, in the context of offerings, something like "condition which permits something" or maybe "prerequisite." Where can I read a good introduction to this term/concept? I get the gist from learning the mishnayot and commentaries where I found it, but I'd like to get a sense of this concept on its own terms, since it's apparently a ubiquitous one in discussions of offerings.
You are right when you you said permits something. The sprinkling blood of the sacrifice permits the eating of eat. There are different מתירים for different things. In order to eat the Menachot one must burn the Kamitza. I'm not sure if this is correct but an live animal is forbidden to eat because taking a limb from a live animal but the Shecting of it is a מתיר. Hope this help.