Back in North Dakota

So working with the Air Force, I've noticed a few things. But there is this one thing that is totally bugging me that I know that my brother, Jake, can answer really quickly. These guys always ask me if I'm "clear" or "secure". I have no idea what it means to be "clear" or "secure" unless it means my level of acne or my mental state. What condition would cause me not to be "clear" or "secure"? If I have a gun, does that make me not secure, or dare I say, insecure? If I have malicious plans, does that make me not clear? Just wondering. Anyways, I just say "yes?" and they let me in. I must have a very innocent face. I'd make a good con artist if I wasn't a Christian, ha ha. Eh, ok, maybe not.