That's an ICAO/hex code. The transponder ping always includes that information, and then tar1090 tries to look it up in a database for other information, like the registration. If it can't find it in the database, then it just displays the code.
Codes that begin with AE are military aircraft, which don't appear in any official (e.g. FAA) database. Enthusiasts have built somewhat ad hoc databases that include many, but not all of the military ICAOs.