Mutation testing is a technique that rewrites source code in small ways to remove redundancies. It introduces artificial faults to test whether tests can detect these faults. It benefits developers by bringing new kinds of errors to light that conventional testing may miss. There are three main types of mutations: value mutations change values to test for errors; decision mutations alter conditions to check for design flaws; and statement mutations modify statements through deletion or duplication to simulate copy/paste errors. Mutation testing is a powerful method for detecting hidden defects.