Overload is perhaps the most fundamental concept you must understand in order to stop exercising and start real training!
Good news: Most Strength and Conditioning professionals make this harder than it has to be -- stressing the system results in an initial shock and a decrease in performance -- recover appropriately and the system's resistance to the initial stress has increased. This is what we commonly refer to as gains (or gainz if you prefer). Continue to modify the exercise prescription so that it is "harder" the next time than it was the last time -- this can be done by increasing the number of repetitions, decreasing rest time, or increasing the load. Why do I need to know this?
Overload is a key first ingredient to getting bigger, faster, stronger, etc.
You must train with enough intensity to elicit a stress response (a weight that is heavy enough or a speed that is fast enough)
Your body's response? Phase 1 - Alarm - an initial decrease in performance
But with proper recovery, you increase your resistance to the initial stress - aka you get better.
"Understand the G.A.S. & unlock your potential"
If you're the guy to the right using a smith machine to bench press a weight that is less than 60% of your 1RM, and you routinely bench this same weight, you have not overloaded the system enough to elicit an adaptation (muscle growth, improvement in endurance, etc.)
If you wind up like this guy at the end of every workout, then you are probably just exercising and not actually training... smart training rarely ends with this result.
Sometimes the proper prescription is a rest day, several rest days, or a scheduled de-load.
Keeping it Simple...
"As to methods there may be a million and then some, but principles are few. One who grasps principles can successfully select their own methods."
Scroll left or right to check out the next principle.