please explain what a challenge coin means to non military folks out there. and how one gets one.
Generally, it's a coin or token representing any particular base, section, unit, or what have you - almost always military and sometimes from fraternities. They are usually, traditionally, offered as a token of esteem or for a job well done. Some times they are available for purchase. Normally carried by unit members showing they are a part of that particular unit.
The challenge? When challenged, the members, or all present, must produce the coin upon request or pay some sort of penalty - usually a round at the bar. The cool part is that if everyone HAS their coin, the challenger then must buy the drinks...
It's a moral builder and very cool tradition...
A google search for challenge coins will produce zillions of examples.