A set is a certain number of reps of one exercise. If you performed 10 push ups, that would be one set. You take some rest and then repeat it to complete another set.
A superset involves two exercises done together with no rest in between. For example, you might perform 5 pull ups followed by 5 push ups. You then take rest, and repeat the two exercises until you've completed the desired number of sets.
You can check out what a workout made up of supersets would like in our ring workout article.