PPM, or procs per minute, refers to the frequency with which certain effects can occur, mostly passive item triggers. Procs usually have a percent chance that they can occur, however if the rate of what triggers that effect is greatly increased, these effects can be abused. As a result, Blizzard limits the number of times these effects can occur to a fixed number of times per minute.
The PPM rate is not a hard, fixed quantity, and it might be possible to see the actual rate be higher or lower, but it should never average above the rate. A similar concept, internal cooldown, is used when something truly is not supposed to reoccur in a short amount of time or even be active constantly.