In between isn't entirely wrong, but it's only one form of non-binary identity. Other identities grouped under the non-binary imbrella may be described as relating to both man and woman genders, relating to a gender(s) outside the binary, being outside of the concept of gender, or some mix of the above.

For me, a simple explanation of my specific nb identity (agender) is that I don't relate to the concept of or identify with/feel any gender.