Combinatorics is the study of how to create combinations and permutations of objects. It goes hand in hand with set theory. In the context of voting theory, combinatorics is often used when analyzing Proportional Representation and PSC to determine which candidates or groups of candidates should win. In this context, it can be used to find all possible winner sets and compare them to each other.
A good combinatorics calculator is at https://www.mathsisfun.com/combinatorics/combinations-permutations-calculator.html.