The phrase "in passing" is an adverbial expression that means something is mentioned casually or briefly, often while talking about something else. It's like saying that the information is not the main focus of the conversation but is included incidentally.
By the way: This is often used to introduce a new topic or point that is not the main focus.
On the side: This can imply something done in addition to the main activity, similar to "in passing" but usually refers to actions rather than words.
To sum up, "in passing" is a useful phrase to describe information that is casually mentioned while discussing something else.