Now imagine that your entire field of view is limited by the bounds of this finger frame. Anything outside of the frame is not visible to you.
If you wanted to see a larger (wider) field of view, you have two options.
The first is to move the finger frame closer to your eye. You'll still see the bounds of the frame, but you'll be able to see a large field inside the frame.
This is the analog of moving from the 40 mm flange depth of DSLRs to the much shorter 16–20 mm flange depth of mirrorless cameras.
The throat size of a lens mount is the inner diameter of a mount, minus the tabs that are used to mount lenses.