Lots of people find this confusing. Think about it like this: I do one bicep curl. Then I do another. Then another. I keep doing bicep curls, counting the number of times I go through the complete action of lifting the dumbbell and lowering the dumbbell until I have counted 10 times. I have lifted the dumbbell and lowered it ten times —that is one set of 10 repetitions (reps). Then I rest.
After a rest, I do 10 more bicep curls. Now I have done 20 repetitions in total, with a rest in the middle. That completes two sets of ten repetitions, or in exercise lingo, two sets of 10 reps.