Mocks replace slow, expensive, random, or external systems with controlled behavior. They make tests stable but can hide real integration problems if overused.
ELI5
Think of Mock as a simple recipe for doing the work better. A fake version of a dependency used in tests or local development.
In practice
Use it when you need a repeatable method instead of guessing from vibes. In practice, define the owner, input, output, and failure mode before you rely on it.