Not meeting any one of these four areas and the individual will not receive realistic results. PRE is a concept of continually adjusting for an increase in weight, number of reps or sets. Our body is great at adjusting to changes in the environment. If you use 3 sets of 10 reps with 25 pounds for a curl exercise after 3 or 4 weeks the weight would no longer be enough stimulus to elicit change. You would have to either increase the weight, number of reps, or sets to continue to improve in strength.
Overload is the principle of more work than the muscle normally encounters.
For example, if you are in-active all winter and you suddenly decided to clean out the garage, you would most likely encounter an overload. Overload is more resistance than you experience day to day. With strength training, you must increase the amount of work performed to continue to make improvements. Specificity is the concept of strength training the specific muscle group that the performance is desired in. For example, if you need strong legs, arm curls will not help.