A superset is two exercises done back to back with little or no rest between them, followed by one normal rest period. The classic version pairs opposing muscle groups: a chest press straight into a row, or biceps curls into triceps pushdowns. One muscle rests while the other works, so you compress two sets into the time of one.
Why it matters
Time is the number one reason people quit training, and supersets attack it directly. Research on agonist-antagonist pairings shows you can cut session length by 30 to 50 percent while keeping nearly all of your rep performance. That's a rare deal in training: almost nothing is free, but this comes close. The catch is pairing. Stack two exercises that share a muscle (bench press into overhead press, say) and your second exercise suffers badly.
How to use it in training
Pair movements that don't compete. Push with pull, upper with lower, or a big lift with a small unrelated one. Do exercise A, walk straight to exercise B, then rest 60 to 90 seconds and repeat.
Keep heavy, technical lifts like squats and deadlifts out of supersets. They need full rest and full focus. Save the pairings for machine work, dumbbell work, and accessories, where a little extra fatigue won't hurt your form. In a crowded gym, pick two stations close together so you're not losing your equipment mid-set.