Unittest is a built-in testing framework in the Python programming language that helps developers verify that their code works as intended. It allows users to create test cases, which are small programs that check specific functions or methods for expected outcomes. By running these tests, developers can identify bugs and ensure that changes to the code do not introduce new issues. The framework supports various testing techniques, including unit testing, where individual components are tested in isolation. Unittest provides tools for organizing tests, reporting results, and managing test execution, making it easier for developers to maintain code quality throughout the software development process.