Test data is the data that is used by 
the testers to run the test cases. 
Whilst running the test cases, 
testers need to enter some input data. 
To do so, testers prepare test data. 
It can be prepared manually and also by using tools.

For example, To test a basic login 
functionality having a user id, password
fields. We need to enter some data
in the user id and password fields.
So we need to collect some test data.