What are forced reps?
Forced repetitions, or forced reps, is a technique that will allow you to do a few more repetitions that you wouldn't be able to do alone.
With forced reps you will need a training partner to help you.
Basically, you'll do as many repetitions as you can on your own and, when you get close to failure, your training partner will give you a little help with the movement so you can do 2 or 3 more repetitions.
You put 100kg on the bench press and do 5 repetitions alone, however, you're practically at your limit and you don't know if you'll be able to do the sixth repetition.
Your training partner gives the bar a quick tap to help you do that extra rep and possibly one more.