Pulses per Quarter Note refers to the timing resolution of a MIDI sequencer. It refers to the number of subdivisions within a quarter note that a MIDI event could possibly start on. Nowadays resolutions are high enough (960 ppqn and more) that most sequencers will accurately reproduce what is played into them, compared to early sequencers that had resolutions of 96 ppqn, and sounded mechanical.
(See MIDI)