A playlist is a timed sequence of layouts (if you’re looking to schedule based on date/time rather than duration, you should check calendars).
It is essentially an ordered list of items, where each item is assigned a Layout (picked from the ones you will have created) and a Duration (in Hours, Minutes, Seconds).
When you update a layout included in one or many playlists, the changes will automatically be reflected in the playlists within a few seconds.
A playlist is always looped, so if you assign a 4 hours long playlist to a screen, it will repeat itself every 4 hours.
When creating your playlist, you will notice Wallaboard conveniently calculates the total duration for you.

Check this short animation showing how a playlist works: