FOV is the extent of the observable game world that is seen on the display at any given moment. It is typically measured as an angle, although whether this angle is the horizontal, vertical, or diagonal component of the field of view varies from game to game.
Don't know what FOV you play your game at?
Check the Advanced Options, and look for this!

As indicated by my slider, I play maxed out at 120. It's a pretty big screen.
I was quite fond of 90 for most of my Blockland days however.