This video introduces the units modulo \(m\).

Units modulo m - Let \(m>0\) be an integer. Then \(U_m\) is the set of congruence classes modulo \(m\) which are invertible. \(\overline{a}\) is invertible if there exists \(\overline{b}\) such that \(\overline{a}\overline{b} = \overline{1}\) or in other words \(ab\equiv 1\bmod{m}\).