A digital signage CMS is the software that holds your media, decides what plays on which screen and when, and gets it there. It is the difference between a network you operate and a set of screens someone visits with a USB stick.
A web CMS and a signage CMS solve different problems, which is why general advice misleads. A signage CMS schedules against real time and real places, splits a screen into zones that update independently, keeps playing when the network drops, and tells you which of four hundred devices went dark overnight. Publishing is the easy part; the fleet is the product.
The caveat is that "CMS" is where vendors hide the gap. Anything that uploads a video calls itself one. The questions that separate them are unglamorous: what happens to a playlist when a single asset fails to download, can a regional manager publish to their own sites without touching anyone else's, and does the schedule survive a device rebooting at 3am.
OmmaSign is a signage CMS: what it actually does is the concrete version of this entry, and the blog covers how a CMS changes a signage operation at length.