It is stand-alone testing of the smallest unit of software. This unit can be either a function or a small block of code inside a function. Unit testing does not check the serviceability of the software to the user.
And even if unit tests are completed successfully in a project, this does not mean that the products work smoothly on the end-user side. At this point, the main purpose and function of Unit testing are for technical documentation. Unit Testing is the first level of software testing and comes first integration testing. Unit Tests are coding and run by the developers themselves.