Supersets are sets where you combine two (or more) exercises. So you first perform a set of the first exercise and then start the set of the second exercise. There is no pause between the two sets. The exercises are performed directly one after the other. After you have completed both sets, or the superset, you take a break.
Supersets are a good intensity technique to overcome a training plateau. Furthermore, training with supersets is usually quicker than training both exercises separately and benefits your cardiovascular system.
There are two different types of supersets: agonistic (same muscle group) supersets and supersets with different muscle groups. The latter includes antagonistic (opposite muscle group) supersets.
With agonistic supersets, you train the same muscle group with both exercises. For example, you could do biceps curls followed by hammer curls or bench press followed by a flying variation. The sequence of movements and the function of your muscles vary, but the muscle group being trained remains the same. This way you increase the metabolic stress in the muscle and set strong stimuli.
You can also do supersets that combine exercises working various different muscle groups. The most common type of supersets like this are antagonistic supersets. With these you train antagonistic muscle groups. These are muscle groups that are opposites of each other. For example, biceps and triceps, chest and back, or quads and hamstrings. So for example, you could do a superset where you first do biceps curls and then perform the triceps press.
Doing supersets with exercises that use different muscle groups can save you time. Also, the first muscle group recovers while you train the second. For example, if you combine a biceps exercise with a calf exercise, your biceps will recover while you train your calves.
Training with supersets is generally best for more advanced athletes. As a beginner, you should not use this intensity technique. Focus instead on the perfect execution of each exercise and build basic strength. Supersets would overload your central nervous system and are therefore not suitable for starting strength training.