Unidades, en castellano = units, in english. And indeed: the only two units in are the residue classes 1 and 5.
In a field every NON-ZERO elements has an inverse.
Of all the rings of residues classes modulo n, the only ones that are fields are those with n a prime number, since then for any non-zero element m of it, which has an integer representative , as (m,p) = 1, we can write by the Euclidean algorithm , and thus a (mod p) is the mult. inverse of p.
You better get a good algebra book and try to read this there since otherwise it'll be pretty hard for you to grab all this stuff.