A digital menu board is a screen — usually a TV mounted behind the counter or drive-thru — that shows your menu the way a printed board would, except the content lives in software instead of on paper or a printed panel. Instead of ordering new vinyl or printed inserts every time a price changes, you edit the layout once and update items from a dashboard (or, in the better systems, straight from your point-of-sale).
The category covers a wide range of setups: a single screen showing a static image that only changes when someone manually updates it, all the way up to a fully synced board where marking an item sold out in your POS grays it out on screen within seconds. Those two ends of the spectrum behave very differently day to day, and it's worth understanding the difference before you shop, because "digital menu board" alone doesn't tell you which one a vendor is actually selling.