[Solved] Can someone explain to me this works?

Recursion: simplify the problem, solve the simpler one

Example: the multiplication of all the numbers from N to 1
1 * 2 * 3 * 4 * 5 * 6 * … * N

Simplification: the multiplication of all the numbers from N to 1 is N multiplied by the multiplication of all the numbers from N - 1 to 1 which is simpler than the example
N * 1 * 2 * 3 * 4 * 5 * 6 * … * (N-1)

in pseudo-code

factorial(N) = N * factorial(N - 1)

but with the simplest case in code

if (N == 1) return 1;

Put these statements in a function and you have the solution.