When do you use "et al."?

Et al. means and others and there are times when you must use it in your in-text references (APA) or footnotes (Chicago).

Chicago Style:

For works by four to ten persons, in a footnote, only the name of the first author is included, followed by et al. with no intervening comma. 

APA 7th Style:

For works by three or more persons, give the name of the first named author only followed by "et al." for in-text citations. 

There are multiple examples in the Referencing Guides - see the Author section in both the Journals and Books pages.