Field of view can be described in a variety of ways.
It can be explained as "the side-to-side measurement of the circular viewing field or subject area.
Or, alternatively as "the widest dimension from left to right that you can see when looking through" the optical device.
The specification that you see with all optical items is usually measured by the width in feet or meters of the area visible at 1000 yards or meters. Alternatively, it can be measured in angular degrees, denoted as Value° (example: humans have 180° field of view or basically the half of a circle). A wider field of view is desirable for many reasons, including but not limited to: following fast moving action, and when scanning in denser backgrounds (grasslands, woodlands, etc.). Note that when magnification is increased, the field of view narrows (sometimes considerably).
The relationship between Field of View (FOV) and Magnification is one of the most critical consideration when selecting an optical device, whether binocular, spotting scope or rifle scope. Give serious consideration to the type of activity you are going to be engaged in and make your selection accordingly. As a rule of thumb, a wider field of view is preferable over additional degrees of magnification for most applications.