Field of view, or FOV for short, is how much of the world a player can see at any given moment.
With a camera device, field of view refers to what the camera (and by extension, the player) can actually see. The field of view is represented as an angle, and is measured in degrees. Angles are two lines that join at a point called the vertex. With cameras, the vertex is the lens (virtual in this case) of the camera. The arms of the angle spread up and down (the vertical axis) from that vertex. The higher the number of degrees, the wider the angle, and the more the camera can see.