What is mutation testing and how does it improve test quality?

Mutation testing evaluates test quality by introducing small code changes (mutants) and checking if tests detect them. Mutations: change operators (> to >=), negate conditions, remove statements. Killed mutant: tests fail (good). Surviving mutant: tests pass despite bug (weak test). Mutation score = killed/total. Tools: PIT (Java), mutmut (Python), Stryker (JS). Higher mutation score indicates more thorough tests. Challenges: slow execution, equivalent mutants (semantically same). Use for critical code paths.