#"Primary, secondary, tertiary"# refers to the degree of hydrocarbyl substitution on the ammonia molecule.
Explanation:
Ammonia is #NH_3#. A primary, #1""^@#, amine is #RNH_2#, that is it has one hydrocarbyl substituent on nitrogen. A secondary, #2""^@#, amine is #R_2NH#, and a tertiary, #3""^@#, amine is #R_3N#. A quaternary ammonium salt is #R_4N^+#.