Q factor is the overall width of an installed crankset, measured parallel to the bottom bracket shell from the outside of one pedal insertion point to the other. You can think of it like this: the larger the Q factor, the farther apart your feet will be.
Q factor has long been a measurement of concern for crank manufacturers and cyclists alike. An overly-wide Q factor results in decreased pedal-ground clearance while the rider is turning. An overly-narrow Q factor will cause chainstay clearance issues at some point. Not all Q factors will be compatible with all frames.
Generally speaking, Q factor is kept as narrow as possible without impacting frame clearance. Several developments have pushed Q factor farther from the centerline of the bicycle, primarily:
Q factor is not usually advertised by crank manufacturers or QBP. However, some crankset models have recently been made available in multiple Q factor variations. For these cranksets, QBP will identify Q factor width in the item description.