Each problem has a set of tests. Once you think your code is working correctly (and you've used the run button to test it yourself!) you can submit your program to the marking system for testing.
You can run the marker as many times as you want.
There are two types of tests. The first set are used to decide whether your submission will be marked as correct and you will receive feedback from the marking system on these tests.
The second set are used to award points. In order to receive points for your submission, you must pass all of these tests. Note however that feedback from these tests will not be shown in the output from the marking system.