+P loads are higher than standard pressure loads. Industry standard (SAMMI) pressure standards are usually well below the point the cartridge case fails and the performance of the round can be increased by increasing the pressure (more powder). Strong modern firearms (steel frames, heavy barrels, etc.) can usually handle higher than industry standard pressures and +P gives added performance. Better to stay away from them if you're not absolutely certain of your firearm since it may damage the weapon or wear it out rapidly even if it doesn't blow it up. I never fire +P, I just get a more powerful gun if I feel the need for more performance.
Excellent explanation. I was just teasing, but with the FR crowd, someone knows the answer. The Right answer.