Progressive Overload is a must when you want to gain muscle/size. It's a simple concept but one people forget or misunderstand. Here is an example of linear progression (which is one variation of progressive overload).
If you do 3x8 program, you start with a weight you can do 3 sets and 8 reps with it (let's say 20lb dumbbell on curls). Then next workout take a bigger weight (either a closest dumbbell or add 5lbs to the barbell). You probably will do 3 sets and 6 reps maybe. Ok. On the next workout you should be able to add a few reps to it. If you eat enough and get enough rest. Then in a week or two you will be able to do 3 sets of 8 reps with this weight. Now is the time to add weight again.
You wont be able to progress for long time with this linear progression (take a week off after two month or even a month of training if you haven't added any reps in 3 workouts for an exercise).
Hopefully this small post will be easy to understand and helpful for new guys just starting to lift.
BTW, maybe more experienced guys could elaborate more and post better explanation of it.