If you are doing bench presses and you casually do each set without grinding out the last few reps that the load is too easy for you and you need to add more weight to the bar. (fitnessandpower.com)
The greatest muscle gains will come with the last few reps that you crank out even when your mind thinks that it can't do anymore. (fitnessandpower.com)
Having a spotter can be very useful when you're struggling to complete those last few reps, or when you can't determine whether your form is OK or not by yourself. (fitnessandpower.com)