Testing software verifies that the software works as you expect and as you want it to function. Testing is vital to the full software developer life cycle. Without testing software you write you cannot confirm that is functions as you expect, reading and understanding what the code should be doing is only half the battle. If you have ever written any software, say for example a game, and run the game and played parts of it then you have tested your software. You may not have tested it fully, but any testing is better than zero testing.
There are different levels of testing that can be run against software you write. Running the software up and using it yourself is testing, giving it to other to use is also testing. You can also write other code to test the code you have written, this may not make sense initially but it is an important aspect of testing software. The additional ‘test code’ you write it not included within the software you release, it is only used to verify the code functions as you expect.
To summarise, testing is a vital aspect of software development. You can write additional code to test the existing code, you can run the program and test it as an end-user would use it, you could release a pre-release (or an alpha) version of the software for other people to test, or many other ways of verifying the software functions as it should.