A restrictive or defining clause adds essential information about the subject. This information cannot be eliminated without losing meaning, and it is not set off by a comma or put in parentheses. Either that or which is used as the pronoun introducing the clause.

The book that explains these concepts was published in 2012.

Here, the restrictive clause specifies which book is being talked about. We readers might not know yet which book is being described, and the specific information that the book ‘explains these concepts’ helps us to understand exactly which book the author is referring to.

[Compare to a non-restrictive clause, like “The book, which explains these concepts, was published in 2012.” Here, we readers already know which book is being talked about, and the fact that the book ‘explains these concepts’ is only extra information that could be left out without altering our understanding.]