It’s a fetish where one character eats another character (or in some variants ends up completely inside the other character in some other way), generally having the eaten character swallowed whole and alive. For obvious reasons it’s one of those fantasy-only sort of fetishes that only really exists in art and writing, so how realistic it’s portrayed varies a lot and it may or may not be fantasized as dangerous or safe, or pleasant or unpleasant. Seems somewhat more common with furries for some reason though still uncommon enough that most won’t be into it