It is how much you can see.. Think of it this way, when you zoom in with a scope in first person shooters, the FOV decreases. You see less on the sides and it focuses in on the center.
That's low FOV. Increasing the FOV to 90 means that the screen renders 90 degrees in front of your character, which is pretty natural compared to real life. (though as you know, in real life we have very wide vision, almost up to 180 degrees, but that would look weird on a monitor).
So in essence, if your computer can handle rendering a little bit more on the sides, increasing the FOV to 90 or such will let you see more of the world in front of your characters eyes, rather than being mr magoo
