Field of View describes the vertical and horizontal dimensions in degrees. You will be able to identify the FOV as the black edges surrounding the images. Correspondingly, a smaller Field of view creates a narrower and less immersive experience.
The closer you can get your eyes to the lens the better FOV you will achieve. In all cases, a larger number is better.
Wearing glasses in the headset will lower the overall FOV as space required to allow for the glasses to fit.
IPD also changes the Field of View, especially the horizontal due to where the eyes are positioned relative to the lens apertures.