Unit testing is a software testing method to validate that code works as intended. It involves arranging test conditions, running code called the method under test, and asserting expected outcomes. Well-written unit tests are isolated, repeatable, self-verifying, and timely. Unit testing catches regressions early, allows for refactoring with confidence, and serves as documentation. It is an important practice because it ensures code reliability and prevents bugs from reoccurring as software evolves over time.