Alert refers to ATC/Airport Ops/ARFF classification of an aircraft with an emergency.
Alert 1 is declared when an aircraft has a minor emergency reported of some type.....generally that aircraft will land and taxi off or back under its own power.
Alert 2 is declared when there's an emergency declared for something major, such as loss of a hydraulic system, electrical problems, landing gear emergencyt extension, etc. An aircraft landing out of this generally will have ARFF vehicles staged and following, and will likely not taki back on its own.
Alert 3 is declared when an actual accident has occurred or is known to going to be occurring. Aircraft inbound with an actual inflight fire, a plane that cant get its landing gear extended, etc. ARFF will be staged and ready to go and support units called up and ready.