Answered By: Gabe Gossett Last Updated: Jan 14, 2016 Views: 19163
Square brackets are used around words that are added that are not part of the original quote. For instance, you might have a source that says "Brenda and David went to the store," but you only want the quote to refer to David as a pronoun in your quote. So you should change it to "[He] went to the store."
Brackets can also be used with quotes for explanation for how you changed the quote from the original source. For example, you might write "Brenda and David went to the store [emphasis added]."
See APA Manual page 173 for more information.