the order of a mod n is the smallest power of a that makes it 1 (mod n). Typically denoted as K.
AKA:
\[a^k \bmod n \equiv 1\]It’s pretty much just a guess and check operation, but have no fear! K will be less than or equal to n. If no K is found, then we say the order of a is infinity.
if order(a,n)=phi(n), then a is a primitive root of n.