Nothing is more frustrating than putting in hours at the gym and not seeing results. This is hitting a plateau and the feeling is exactly that; pure frustration.
Basically, hitting a plateau means that your body stops responding to the stress your placing upon it. You stop seeing changes, therefor new results lessen and eventually diminish.
Why does this happen?
The best example I can think of is this: Let’s say you are eating 2000 calories a day. Suddenly you decide to only eat 1800 calories a day. Two weeks go by and your down 5 pounds! Another two weeks go by, your still eating 1800 calories, but you’re still only 5 lbs down. You have hit a plateau. You need to make a change in order to overcome this, such as dropping down to 1600 calories or increasing activity.