A lot of players wonder what FOV stands for, and the "FOV" thing in game's options often brings a lot of confusion. FOV stands for Field of View, and it is, well, the actual field of view. Frankly speaking, it is how much you can observe without changing a perspective (i.e. moving, turning, etc). For example, if your FOV is 90 degrees, imagine a 90-degress angle positioned exactly in the... point, where from you look - in game's is a player's "camera", in real life you can take as a place somewhere between your both eyes (it is much better if you have one eye, then you can imagine that angle right at it). Now, project the sides of that angle indefinitely into the infinity - everything which happens to be between sides of the angle is observed by you, everything else is not. So, the more FOV is, the more you can see. Take in account that bigger FOVs result in a kind of "fish-view", which may hurt you in multiplayer games more than help.
We are talking about horizontal FOV right now, but there is also a vertical one, and in some games, like GTA 5, in game's settings you adjust vertical FOV, not horizontal one. It is important to make a note that they are not separate entities, they are mutually expressed through each other, however it is rather unclear to a regular player what effect changes of vertical FOV have on horizontal FOV.
Just input your game resolution (or just ratio), horizontal FOV you want and press "Count" button.
This calculator is useful for such mods like "FOV mod for GTA 5