Question

What are sets and reps in exercising?

Answer 1

If someone says do 5 reps of 20 push-ups yo do 20 push-ups then take a little break and do that 5 times

Answer 2

Reps is short-hand for "repetitions" so it is how many times you repeat the same exercise in a set.
number of sets in turn is how many time you repeat the whole "set"  So like the example mentioned in the other ans, to do 5 sets of 20 reps for push-ups mean u will be doing 5x20=100 pushups.