Hi, Codeforces Community!

I recently looked to a programm written by a participant in the contest, and I noticed this function, which is finding the greatest common devisor of two numbers.

long long int gcd(long long int a, long long int b) { return (b == 0LL ? a : gcd(b, a % b)); }

But I don't understand how this part (b == 0LL ? a : gcd(b, a % b)) of the function is working. Can anyone explain please?