Two integer numbers are coprime or relatively primes when their greatest common divisor is 1 ($\displaystyle{ \gcd{(x,y)} = 1 }$). This does not mean that any of these numbers is prime.
Two random numbers are coprime with a probability over 60% (the exact number is $\displaystyle{ 6/\pi^2 }$).
When two integers $\displaystyle{ m }$ and $\displaystyle{ n }$ are coprime, it is possible to define the modular inversion of $\displaystyle{ m \pmod{n} }$ and thus the modular division by $\displaystyle{ m \pmod{n} }$.