In MLA format, when a source has two authors, you separate their names with "and" in the in-text citation. For example, (Smith and Johnson 24) - easy peasy, right? However, when there are three or more authors, things get a bit tricky, and you need to use "et al." which is short for "et alii" or "and others" in Latin.
Imagine you're at a dinner party, and you're introducing your friends to each other. If you have two friends, John and Mary, you'd introduce them as "John and Mary". But, if you have five friends, John, Mary, David, Emily, and Michael, you'd introduce them as "John, Mary, and friends" - the "et al." is like saying "and friends" in your citation. For instance, (Smith et al. 34) is the way to go.