I have seen many people are writing unit tests for their java programs. But what unit testing actually mean? What is the purpose of unit testing? What would be the result if we do not perform unit testing for our software?
The really, really short version is that unit tests are inteded to verify the correctness of individual units of a system. If you have a comprehensive test set you're essentially working with a safety net, because as soon as you make changes that break the contract guarded by a unit test, it will fail and you will know something's wrong right away.
Build a man a fire, and he'll be warm for a day. Set a man on fire, and he'll be warm for the rest of his life.