Unit tests are small, automated tests written by developers to verify that individual parts of a program, called units, work correctly. Each unit is tested in isolation to ensure that it performs as expected, helping to catch bugs early in the development process. This practice improves code quality and makes it easier to maintain and update software. By running unit tests, developers can quickly identify issues when changes are made to the code. If a test fails, it indicates that there may be a problem with the specific unit being tested, allowing for faster debugging and more reliable software overall.