Unit testing is a software testing technique where individual units (usually functions or class methods) of a software system are tested in isolation to ensure that they are functioning correctly. There tests are fully automated and should focus on testing the behavior of the unit, typically with a set of predefined inputs and expected outputs. (But there are methods to make those inputs randomized, see property-based testing.)
The purpose of unit testing is to catch defects early in the development process and to provide a safety net for code changes, ensuring that new code does not break existing functionality. Unit testing is an important practice in software development and is often integrated into continuous integration and delivery workflows.
We talk about testing a lot on this blog. Have a look at some existing articles on this topic: