A playlist does nothing until you tell it where to play. In Screenly, you do that with labels.
Rather than picking screens one by one, you give a playlist a label, and every screen carrying that label plays it. Each screen has a label of its own, so you can target a single display. You can also use a shared label, like a location or a purpose, so a playlist lands on a whole group at once. Add the label, and the right screens pick the playlist up on their next sync.
This is what keeps a large network manageable. Assign a playlist to a label that fifty screens share, and all fifty play it. Update the playlist later, and all fifty follow, with no trip to any of them. You are always working with the playlist, never chasing individual screens.
A playlist also carries its own sense of when. As well as where it plays, it can be set to run only at certain times, so a morning briefing loop and an after-hours loop can live on the same screens without colliding. The deeper rules behind that, like priorities and dayparting, are their own subject. Here it is enough to know that timing travels with the playlist, not separately from it.