Firearms Owners IDentification card. In Illinois you must have one to buy, sell, or possess guns and/or ammo. To get one you pay a few bucks and go thru a background check by the State Police.
Apparently it is linked to other files in the State. I know that when my trans died on an Expressway, the State Police officer was more interested in wanting to search my car for hidden guns than in calling for a tow truck. Odd that by the time he got out of his car, he already knew I was a FOID holder / gun owner.
When Illinois LEO pull up a license, I believe the screen flashes if they are are a FOID card holder.