Test doubles are simplified versions of software components used in testing. They replace real components to isolate the unit being tested, allowing developers to focus on specific functionality without external dependencies. Common types of test doubles include mocks, stubs, and fakes, each serving different purposes in testing scenarios. Using test doubles helps ensure that tests run faster and are more reliable. By simulating the behavior of complex systems or external services, developers can create controlled environments to validate their code. This practice enhances the quality of software by identifying issues early in the development process.