Digital signage is any network of screens showing content that is managed centrally rather than at the screen itself. The screen is the least interesting part; what makes it signage rather than a television is that somebody, somewhere, can change what all of them are showing without visiting any of them.
It covers a menu board in a drive-thru, a wayfinding kiosk in a hospital, a video wall in a lobby and a shelf-edge display in a supermarket. What they share is the operational problem: content that has to be right, current, and consistent across places nobody is standing in.
The caveat is that the screens are the cheap part. The cost that surprises people is everything after installation: who makes the content each week, who notices when a player drops offline, who has permission to publish to which sites. Networks fail from neglect far more often than from hardware.
For the longer version, the blog goes into how the pieces fit together and which industries actually use it.