You can find this in the slide editor.
Check out this visual example on how save and apply a layout.
This option is good to make slides with the same layout on multiple sliders too, and it's also good to make backups when you make a more complex slide, because you can save it at certain steps, and if you don't want the changes you made, or something happens, like having a black out during saving, and the slide gets lost, you can easily change it back to its previous state.
You can load the whole layout or just the layers without the slide settings, so if you have a half done slide, it won't remove anything from it, it will just add the layers. You will see this question asked every time you load a layout.
Load whole slide
If you load the whole slide, you will lose every layer and the background settings from your current slide.
Load only layers
This option will leave your layers and background settings and the layout's layers will load as additional layers.
Sets contain your layouts to help manage them better. You can create, rename and delete sets by clicking on Manage.
Save as new layout
This option will save down your current slide's layout into the chosen set.