Stands for Quench-Polish-Quench, a clear Tenifer finish as opposed to the black. Some refer to it erroneously as Stainless. It is not, but has that appearance.
Just make sure you get a German one, identified by the proof marks on the right rear of the slide. To my knowledge, the S&W made slides were all blue, so a QPQ should be a safe bet.
Regards,
James