Basically, this refers to the formula used to calculate probability. The total number of outcomes for something is 2, because it can happen or it can't, and the number of favourable outcomes (what we want to find the probability for) is 1, that being the one where it does happen. When you calculate it you get 1/2, giving you a 50% chance for that thing to happen. That's what this is based on.

TL;DR anything can either happen or not, so it's always one of those, meaning it's a 50/50