Unit testing is considered the first step of testing in the software development life cycle that involves validating each testable part (also known as units/modules/components) of a software application to determine each unit of the application's code works as intended. Unit testing is usually performed in the early development stages of an application by developers and QA engineers.
The primary objective of unit testing is to isolate a code section and test its correctness. It helps uncover early bugs and flaws in application code that can be more challenging to identify in the later stages of the software testing life cycle (STLC).