Your timesheet is the way you track the hours you've already worked. You may clock in and out each day or keep track of your hours on your own. This is how your employer keeps track of the hours you've worked. It's also how Activehours keeps track of how much you've earned.
Your timesheet shouldn't be confused with a schedule (the hours you're going to work in the future) or a pay stub (the hours you were recently paid for). A timesheet specifically tracks how many hours you've already worked, but have not yet been paid for.