The Disjoint Set Union (also called Union-Find) is a data structure that does exactly two things efficiently:
Find: Given an element, find which group it belongs to.
Union: Merge two groups into one.
It answers questions like "Are A and B in the same group?" and "Combine the groups containing A and B." DSU does both operations in nearly amortized time. Without it, checking connectivity or counting components requires full graph traversals each time.