What are non-restrictive clauses?

Definition of nonrestrictive clause

: a descriptive clause that is not essential to the definiteness of the meaning of the word it modifies (such as who is retired in "my father, who is retired, does volunteer work")

A clause which adds extra info into a sentence.

Explanation:

It is a noun which gives additional information about something in a sentence

Example: The kids needed good pencils, which were expensive.

The clause teaches us that the good pencils are expensive.

isn't this axel talking to his uncle?