The Submit server is used in some CS classes as a means of collecting and grading assignments. Sometimes it is simply used as a place to upload papers. However it can also be used to grade programming assignments. You may be given a programming project and asked to upload your code to the Submit server. There it will be built, run, and the results evaluated against a baseline.
If your instructor chooses to use the Submit server system they should provide more information on how it will be used in their course.