What is “illegal possession of concealed firearm”?
Just that. Whenever you have a firearm hidden on you or in your personal effects (handbag, backpack, pocket, etc), and you don’t have a concealed weapon carry permit, then it is illegal. The specifics of this varies from state to state, by the way, so a good thing to do to research your state’s (or city’s) weapon laws would be to visit yourstate.gov and check out the specific law. If someone you know has been arrested on this charge, contact a lawyer too. The charge is serious, and in states with average gun control laws on the books, the offender could be facing prison time.