To simplify: it’s essentially work X hours per week, performing the job duties described for the position, the specifics of which will be supplied to you by your boss (they tell you what to do).

If your boss believes you are performing a satisfactory amount of work for the time spent, you keep your job. If not, or they just don’t like you for some reason not protected by law, you lose your job.

You do what they tell you and hope they are happy with the results.