A non-restrictive or non-defining clause adds information that is not essential. The information could be set off in parentheses, commas, or eliminated entirely without a significant loss of meaning. Usually, either which or who is used as the pronoun introducing the clause.

The book, which explains these concepts, was published in 2012.

Here, the non-restrictive clause simply adds some additional information about the book. We readers already know which book is being described, and the fact that the book ‘explains these concepts’ is only extra information that could be left out without altering our understanding of which book the author is referring to.

[Compare to a restrictive clause, like “The book that explains these concepts was published in 2012.” Here, we readers might not know yet which book is being described (perhaps this is the first time in the text that the book has been mentioned).]