PPQN (Pulses Per Quarter Note, also known as PPQ, Pulses Per Quarter and TPQN, Ticks Per Quarter Note) is the smallest unit of time used for sequencing note and automation events, both by step sequencers and in the MIDI standard. Most MIDI sequencers allow the number of PPQ to be varied for more or less temporal resolution depending on the needs of the performer.
If there are too few PPQ any performance recorded into the MIDI sequencer may sound artificial (being quantised by the Pulse rate) or lose subtle variations in timing that gives music a 'human' feeling. Generally 96 PPQ is sufficient to capture enough temporal variation. However, some musicians like to work with PPQs around 960 or more.
PPQ is a relative measure of time since it is the tempo of the MIDI sequence, set in Beats Per Minute BPM, that defines the length of a quarter note (in seconds) and so the duration of 1 Pulse (or 'tick' as it is sometimes known to programmers).