Et al is an abbreviation short for the Latin phrase "et alii" meaning ‘and others’ and it is used in an in-text citation (with a full-stop at the end) when you referring to a source with three or more authors. It'd be quite tedious to list all of the authors of a source when you might mention that source several times in your work! The image below shows an article with nine authors.
Imagine having to type Piwowar, Priem, Larivière, Alperin, Matthias, Norlander, Farley, West and Haustein (2018) every time you referred to this article! Thankfully, you don't; instead you'd type "Piowar et al. 2018" in the in-text citation and only type out all nine authors once, in the reference list.
