Something that's passing happens very quickly. If you give someone a passing glance, you look so briefly in their direction that you barely see them.
Passing is one of those versatile words with many different uses. A passing reference is a light, brief mention of something, and a passing grade is one that's high enough for you to pass a test. As a noun, passing often means "the end of something," like the passing of an era — while the passing of a person means death. When you talk about something "in passing," you mention it is a casual way.