Ammunition rated +P generates a chamber pressure higher than the cartridge’s published standard, but the level is less than those produced when proof rounds initially torture test a firearm. Increased bullet velocity is the result—an advantage for some self-defense handguns—but not all guns can safely run the hotter loads. In those cases, the added stress can prematurely wear parts, void some warranties or, worse yet, lead to catastrophic failure and personal injury.
Owners of firearms rated for its use, however, can run the ammunition with confidence. Improved performance on target is the biggest advantage. Flatter trajectory is another, although +P is primarily handgun ammo, where reduced holdover at 100 yards isn’t a huge concern. That benefit, however, may be of interest to anyone with a pistol-cartridge carbine.

+p ammo is a higher velocity round than your average non-+p. Most modern firearms can handle +p ammunition but its only recommended in smaller quantities and to be safe always check with the manufacturer. Now you also have +p+ Wich there really isn't any saami standard for this, so this is where if you don't have a firearm made for it things can get dangerous cause it's loaded to max pressures. Honestly unless you have a very very short barrel 3 inches or less with today's advancement in bullet technology +p ammos not really needed so why cause extra wear to your gun by shooting higher velocity loads than its use to. Hope this helps.

Jon, +P means plus pressure. It designates ammunition that is loaded hotter than standard SAAMI Standard pressure. https://en.wikipedia.org/wiki/Overpressure_ammunition