I guess irreducible form? (translated from my language, maybe its actually another word?)
The correct answer is:
We say that a fraction is in "simplest form" if it can no longer be simplified. This means that there are no more factors that can be divided out of the numerator and denominator.
If a numerator and denominator have a GCF of 1, this means the greatest thing that divides both of them is 1. This means that there are no more factors we can divide by, which means there is no more simplification to do; this is simplest form.