What is Training to failure?

Training to failure is pushing a set until the next rep is impossible without breaking your form. It is the highest effort you can give a set, equal to an RPE of 10 or zero reps in reserve. Use it carefully and only now and then.

It can be useful on safe exercises, but it is tiring and raises injury risk on heavy lifts. For example, you could take the last set of lateral raises to failure but stop squats and bench presses a rep or two earlier. Beginners can make good progress without going to failure, and a spotter helps.