It's about as difficult as describing vectors in math. Vectors aren't really "little arrows" or "things with magnitude and direction" (because then functions wouldn't count), I think there's a similarly cryptic saying that "a vector is just an element of a vector space" or "a tensor is anything that transforms like a tensor".

In both cases what they really are is an abstract concept defined by things that satisfy some definitions, but in practice it's easier to just look at a few concrete examples like Maybe.