Collision

Two different strings, same hash slot. Like two different people with different last names sharing a mailbox on a shared apartment line. The postbox doesn’t care. It just delivers both.

A good hash spreads keys evenly, so collisions are rare. A bad hash piles everything into a few slots and turns O(1) lookups into a linear walk.

Two main strategies for dealing with collisions: chaining (each bucket is a linked list) and open addressing (probe the next slot).