As said, +P stands for increased pressure, most calibers about 10% over standard under SAAMI standards.
If they chose the powder well, it will mean increased muzzle velocity, usually about 5%. If they did not, you might only get increased muzzle blast and no more bullet velocity.
It won't increase the wear enough to worry about on a quality modern gun.
Be aware of advertising claims. There are only a few calibers that have SAAMI specifications for +P. There are others that are just overloaded. There is no such thing as a .40 S&W +P, for example.
+P+ stands for greatly increased pressure. There is no standard, it is whatever the manufacturer thinks the guns will stand. It is usually to some government agency specification, they know what issue guns it will be shot in.
I would be very careful about shooting +P+ unless I knew what it was actually meant for.
.38 Special +P+ is good for unlimited shooting in a .357 magnum revolver. That is where it came from, there was an agency that worried about adverse publicity if one of their people shot a perp with a MAGNUM.