Mohamad Ali did not start counting his sit ups until after he was in pain. In other words he kept doing reps until failure. This approach is extremely effective but can be incredibly dangerous. It is especially dangerous for beginners.
Beginners should master their lifting form and breathing first under a rigidly structured fitness program. If you are new to fitness you should focus on mind to muscle connection. This happens through slow full reps, good form, and controlled breathing. Only once the mind to muscle connection has been mastered, then training till failure makes sense.
So what is training till failure? It can be described as doing as many sets as physically possible for an exercise while maintaining impeccable form. This can look a bit different for everyone. My rule of thumb is that if I can do more than 2 reps, I have not worked out till failure.