Readable if and writable if, how to use these fields

I have two tables that I've made only visible to "admin" but I would like to let one other member of the team have access without giving her admin status, can I do this with this with "readable if" and "writable if"?

Example, we have 9 members of the team, 3 admin, 2 editors and 4 base (viewer?  I'm using French version).  The two tables I've created about our accounting I only want the admin to have acces, but we're hiring an accounting assistant that I would like to give access without giving her admin status.

Furthermore, what is the difference between admin, editor and base?  When I search for the answer I keep falling on the same spot in the manuel that doesn't give me an answer: "User management, identities and rights"

Thanks for your help.