Upgrading Crankset: What is Q-factor?

This week after many months debating whether or not i should upgrade my crankset I decided to do it. I had always looked for reasons to postpone it. But finally after missing a couple of races because i only had a 39T chainring paired with a 11-32 cassette, I did it.

Why XX?
I decided to go with the XX because I absolutely love the 39 chainring. If you have a 26" this size I think for mostly flat terrain and quick climbs is the perfect ratio.