One of the last two rules for comma usage...

ESSENTIAL AND NON-ESSENTIAL EXPRESSIONS

An essential expression/element affects the basic meaning of the sentence whereas a non-essential one does not. AVOID surrounding essential elements with commas. Placing commas on either side of a non-essential expression indicates that the information within the commas may be informative but not crucial.

Note: Reserve the pronoun "that" to introduce essential elements and the pronoun "which" to introduce non-essential ones. The pronoun who can introduce both types.

Examples:

Collin Creek Mall, which was built in 1980, has enjoyed nearly two decades of prosperous growth.
The suburban mall that has enjoyed the most prosperous growth in Texas is Collin Creek.
Jane Smith, who is usually never at a loss for words, struggled to remember her lines during her acting debut.
The actress who is usually never at a loss for words is Jane Smith.