Unit tests are small, automated tests that check individual parts of a software program, known as units. Each unit is tested in isolation to ensure it behaves as expected. This helps developers catch bugs early in the development process, making it easier to fix issues before they become larger problems. By running unit tests, developers can verify that changes to the code do not break existing functionality. These tests are typically written using testing frameworks, which provide tools to create and run the tests efficiently. Overall, unit testing contributes to higher code quality and more reliable software.