Testing

Basically, when they talk about testing, it is talking about executing programs and see the result whether it is how we want or not. And testing is not only running the program once but, we need to do many times and follow several steps. First one is to make a test plan.
A test plan is a doc that tell us how many times we need to run over the program in order to test through out the whole program.
Next is the two approaches, which is Code Coverage and Data Coverage. Code coverage, which is an approach that make sure we execute all the statements in the "code", not missing any one. For the Data coverage now is not testing the codes but its about the possible "inputs values"; basically try/input possible values.
Then is the Test Plan Implementation. This involves running program (according to the test plan), execute the result and if the result is not as what the user wanted to be, the user must be able to look at the plan and see the mistakes that they'd made to let the program appeared error, that's why the plan is important when testing. 

In my opinion, "testing the program" is very important also, especially with the plans that you made because, if you test the program with the plans you made and run each one by one, if error occurs, you can always go back and check your mistakes since you have the notes. In addition, if you tested the program with input-ing many possible values, you'll get to know more about the program whether it is what YOU want or not (not working/errors occurred).