I'm reading mathematical gems, Vol.1:
He states the Fermat's little theorem:
If $p$ is a prime number, then for every integer a, the number $a^p-a$ is divisible by $p$.
And then there's an addendum:
Actually he stated the equivalent theorem: If $p$ is prime, then $p$ divides $a^{p-1}-1$ for every integer $a$ that is relatively prime to $p$.
I didn't get the meaning of the bold part.
What's the meaning of this?