Sources of light very far away (at infinity) emit rays that are (and stay) parallel to each other.
Shown are two separate sources at infinity, that emit rays forming an angle between them.
This angle is the angular separation between the two sources.
The observing eye is "relaxed", i.e. focused at infinity.
How "big" a distant object appears to the eye depends on the angular separation only.
(A mountain appears just as big from the fifth floor of a building as it does from the first floor.)