Beacons are small Bluetooth Low Energy transmitters that broadcast an identifier, letting a phone determine it is near a specific place indoors, which GPS cannot do reliably inside a building.
The promise is precision: not "in the shopping centre" but "in front of this shelf". That supports indoor navigation, aisle-level offers, and analytics about where people actually go.
The caveat is the one that has defined the technology's history: it requires an app the visitor has installed and granted Bluetooth and location permission to, and left running. That is a very small fraction of any venue's visitors, and the beacon estate has to be maintained (batteries replaced, positions mapped) regardless of how few people it reaches.
For venues with a genuinely used app (an airline, a large grocer, a stadium) beacons can pay. For everyone else, QR codes reach every visitor with no install, no permission and no battery, and the honest comparison usually ends there.