An insertion is when you put something into another thing, like the insertion of a key into a lock, or the insertion of a comment into a conversation.
You probably notice that the noun insertion contains the verb insert, meaning “put in.” An insertion is something that’s put into something else. It can be used literally to describe, for example, the insertion of a page in a binder, but it also can be used more figuratively. For example, the insertion of the president into office happens every four years. And your brother may constantly make insertions into discussions he knows nothing about.