Dynamic backgrounds are procedurally generated animations that add motion to slides without significantly increasing the size of your Keynote file. There are a variety of preset dynamic background styles you can choose and customize. If you use the same dynamic background style on more than one consecutive slide, the animation plays seamlessly when you change slides. You can save a custom dynamic background style so you can use it again later.
Tip: or next to the style thumbnails, then click Add and edit slide layouts in Keynote on Mac