Think of your muscle after it finishes a set when it is fully exhausted. It doesn't necessarily mean that the muscle can't work anymore - you can still push a lighter weight to take the muscle further.
So what are they?
Drop sets are three or four sets of the same exercise repeated without rest. After each set, the weight is reduced. Drop sets work deep into the mitochondria muscle and, trust me, you will feel the burn afterwards. Here are two ways to do drop sets depending on your workout.
8, 8, 15 reps finishing on the lightest weight for 15 reps
6, 6, 8 reps finishing on the lightest weight for 8 reps
So why the drop set?
They are great if you are short of time and ideal to shock your muscles if your body is plateauing during your workout.
Don't skimp on warming up with 20 reps before you begin. These should be done with about 50% of the heaviest weight you intend to lift to make sure the muscles are fully warm.
One last tip?
Have all your weights lined up so that you have no delay in your drop set.
Ready? Set. Drop.