Unit testing is a software testing technique that involves breaking down a piece of code into small units and testing each unit individually to ensure it works as intended. The main idea behind unit testing is to isolate each unit of code and test it in isolation by replacing external dependencies with mock objects or stubs. This helps catch bugs early in the development process, making them easier and cheaper to fix. Writing effective unit tests requires best practices such as keeping tests small and focused, writing tests before code, using descriptive test names, and testing edge cases. Automated unit testing tools like Codium.AI can assist in generating test cases, analyzing code coverage, and identifying areas of the code most likely to be affected by changes. While unit testing can complement manual testing, it cannot replace it entirely, as manual testing can identify usability issues, performance problems, and other issues that might not be apparent through unit testing alone. Implementing unit testing strategies early in the development process is crucial to ensure the quality and stability of code.