Proof and explanation that $\gcd(A, B) = \gcd(B, A \mod B)$

Question

First I'm not studying mathematics and so I need the simplest answer as possible(any example will be appreciated). I was reading a book about cryptography and saw the following equation $$\gcd(A, B) = \gcd(B, A \bmod B)$$which I can't understand at all. But I understand this equation if someone can use it to give a simple answer $$\gcd(A, B) = \gcd(B, A - B)$$