Screesets are presets for the windows inside REAPER. Different tasks require different windows and tools in view and by saving screensets you can instantly jump to the ideal layout instantly as you change tasks.
In this video I show the 5 window layouts I use, and how to save a new screenset.
Access the screensets window from the View menu.
I like to have my screensets load with 1-5 on alpha keys and save with Shift 1-5.