This is fairly long, as it's written with the wiki in mind. But atm the wiki is glitchy, so I'm posting it here.Intro:The 'field of view' in a game controls how much of an area in the game space you can see. With the -fov parameter, you can actually set this paramter to see more or less of the area.
This is sort of akin to the width of a window; if you stand and look out one window, then go to another, wider, window and stand the same distance away, you'll be able to see a wider area with the wider area.
Note that field of view is dependant on your distance from the object as well; if you've ever seen a sequence in a movie or TV show where the background seemed to zoom in or out, while characters remained the same size, the same principal applies
Freespace 2:The field of view in Freepsace 2 is actually set incorrectly to make things appear to scale. It's actually set too large. This is on purpose; by doing this, Volition wanted to make the game easier and more fun to play. Humans have a field of view of about 135 degrees; but most people don't play with their face pressed up against the screen, so their fov is restricted by how much area the computer screen takes up. (See attached diagram)
Figuring out your FOV setting:Figuring out what your FOV setting should be depends on two factors; the width of your monitor, and how far away your face is from it. You'll probably want a ruler of some sort (The kind you measure with, not royalty or GWB). If you're thinking you can cheat because you know the size of your monitor, think again - the size of a monitor is its measurement diagonally,
not its width or height.
Once you've figured out how far away you sit from your monitor, and how wide it is, you'll need to do a little math. Go to
this site and enter the following (Replacing "monitor width" and "your distance" as appropriate):
atan((monitor width/2)/your distance)*2
Now, add the result after "-fov" in the custom flags field of the launcher. For example, "-fov .1786". Start up a mission in Freespace and you should instantly notice the difference.

You might want to tweak the value some, as it can be a bit uncomfortable to play with a small FOV.