Testing is an important aspect of any Software Development Methodology .If you would you read about testing we have different types of testing conducted on different application models:
Unit Testing : Refers to test which are conducted on all the units as one.For example,entire block is being tested as one unit or entire chain is tested as one unit.We have to write the test cases to check if the software or application we're writing will be efficient to all the number of inputs by the users.
Consider a simple example of a calculator : We write a program to add two numbers .Now we will conduct different test cases with n number of inputs like adding two positive numbers/adding two negative numbers/adding a fraction and check in all those scenarios if we're getting the accurate answers from the program.If particular test is unsatisfied we can go ahead and improve the code.
We also perform black box /white box testing on various modules.Its interesting to learn.You could google these concepts .