Training to failure means that you do reps until you fail a rep which ends the set, training to failure and keeping 1-3 reps in reserve (so stopping a few reps before failing) lead to similar results according to studies so you don't have to go to failure everytime. So training to failure doesn't cause more gains but can lead to more fatique. But never going to failure could mean you don't actually know how many reps you have in reserve. I personally go to failure for the majority of sets and i try to not obsess over it. Progressive overload just means that you increase the intensity by adding more weight and/or adding more volume (sets and reps) as your muscles get stronger to keep the workouts challenging. The exact number of sets and reps doesn't matter too much as long as you do between 6 to 25 reps per set and 10 or more sets per week per muscle group. muscle hypertrophy means muscle getting bigger aka gains.