A liminal being is like being in between two things. Imagine you are in a doorway, you are not fully inside the room, but you are not fully outside. You are kind of in the middle.
A liminal being is like that, it's like being in a middle state, between two different things. It's like being in a transition stage, not really belonging to one side or the other.
So let's say you were a caterpillar and you wanted to become a butterfly. That stage in which you are in a cocoon, is a liminal being. You are not a caterpillar anymore, but you are not a butterfly either. You are in the middle, in a transition stage, waiting to become a beautiful butterfly.
In other kinds of stories, liminal beings might be creatures like mermaids, who are not quite fish, but not quite human either. Or like a werewolf, who is not quite human, but not quite a wolf either.
So a liminal being is like a state of being in between two things or two worlds. It's kind of a mysterious and magical in-between state, where new things can happen, where transformations are taking place.