Modular multiplicative inverse

A modular multiplicative inverse is a special number that can be multiplied by another number to make one. For example, if 4 is the modular multiplicative inverse of 15, then 15 x 4 = 1. This means that 15 and 4 can be "inverses" of each other. It's like a secret number that if you multiply it by another number, you get 1.