The terms Header and Footer typically come from word processing programs. These denote repeated elements that show at the top and bottom of every page. Headers and Footers work similarly on PowerPoint slides: the Footer is a line of text that usually appears at the bottom of a slide. By default, the footer with one or more of these three placeholders appears on every slide in a presentation, but you can change that as required. You can also move the content within the Footer area to the top of your slide. By doing so, you change a Footer to a Header!
Learn how to add Headers and Footers to slides in PowerPoint 365 for Mac.