In case it hasn't been explained already (and even if it has, because I'm bored and we need some levity), I'll explain "shotgun" for our international friends.
"Shotgun, "calling shotgun", or "riding shotgun" are all references to riding in the front seat, next to the driver (as opposed to being stuck in the back seat, possibly squeezed in with several other passengers). It's usually called out if you want to sit up front and avoid the crunch.
The term comes from the American old west, when stagecoaches travelled long distances through dangerous territory, often carrying valuables, mail, and passengers.
To ensure the safety of the stage, an additional stage line employee would ride up on the outside front seat, next to the driver, carrying a shotgun. His job was to look out for danger, and use the shotgun for defense while the driver was allowed to stay focussed on driving the stage. The position was referred to as "riding shotgun"...