Maximal Field of view is the range of your visual area, for rifle scopes its measured at 100m. For example, if a maximal field of view is 13m/100m, means that you have on 100m a maximal view of 13m. Rifle scopes with Variable magnification (click for the info in specifications), the maximal field of view is meant when using lowest magnification setting. The higher the magnification the smaller the field of view.
Field of view is an important feature when choosing a rifle scope for your activity. A larger field of view will give you wider view/image, which is an important factor when you are aiming or hunting moving targets. Opposite, smaller field of view gives you narrower view/image, this option is more suitable for non-moving/static targets. Better quality rifle scopes have larger maximal field of view.
Just for information, parameter of lens diameter doesn`t give you a info about the field of view. Lens diameter has influence only on the brightness. For example, you can have 2 rifle scopes that have for example lens diameter of 56 mm, but they can have different maximal field of view.
Source: Revija Lovec