ok i found a reddit comment saying what it is and yeah, i think its basically just like an autism thing (not that thats bad)
heres what it said
“It’s not about acting like other animals (side-note, humans are a kind of animal), it’s about an inner identity. Take a moment to visualize yourself, not restrained by your physical appearance but rather without restrictions. For me, that gives me something that is not human. I feel nonhuman phantom limbs. When I’m stressed and anxious, my instinctive response is to hiss. I sometimes find myself walking on the balls of my feet/my toes as though I were digitigrade rather than plantigrade (and my mother always said I walked oddly and she blamed herself, but I would imagine that if all humans needed to learn to walk “properly” by walking on other people’s feet, humans would not know how to walk “properly” in the first place) and while this is difficult, it also seems to be just as natural to me as walking in a more typically human manner. I sometimes have a strong urge to make sounds that humans don’t seem to make (typically something akin to a growl/whine but with varying emotions). These things are not chosen actions, they just… Exist as part of me. Maybe my brain is wired a little weird, maybe my spirit remembers being in other bodies, whatever the explanation, all I know for sure is that I do experience these things as natural parts of my existence, and labels like therian, alterhuman, and otherkin suit me because of that, they’re descriptive of my experiences of my existence as someone who, while inhabiting a human body, doesn’t feel internally human. ”

someone else compared it to being trans (in a really bad way)

“So the way I’m gonna explain it is very taboo but at least it’s a short easier way to understand it.

Think of a trans person. They know in what kind of body they currently are, but they feel like they shouldn’t be in this body, they know they are in one gender’s body but they feel like they should be in the other gender’s body.

It’s the same thing for therians : they know they are in a human body, but they feel like they should be in an animal body.”