In graphics software, sometimes we saw "field of view" for camera. Can someone explain what does it mean?
The field of view (in the gluPerspective call) is the angle in degrees between a plane which passes through the camera position and the top of your screen and another plane which passes through the camera position and the bottom of your screen.
Edit: Since you need not have a full-screen viewport: Any line which is projected to (x1, 0)-(x2, 0) lies in the first plane, any line that is projected to (x3, height)-(x4, height) lies in the second, where height is the height of your viewport in pixels.
The field of view is normally the horizontal field of view.
This is the angular extent of the observable world Wikipedia
The larger the field of view the more of the scene you can see, but each bit will be seen at a lower resolution.
The vertical field of view is related to the horizontal by the aspect ratio of the view. A square view will have the same horizontal and vertical FoV. A landscape view will have a smaller vertical FoV while a portrait view will have a larger vertical FoV.