The primary funtion of Macdill Air Force Base (besides running the war in Afghanistan) is refueling. They run an entire fleet of these big refueling tankers and fly them around the world to provide support to the rest of the Air Force. There is a certain tactical advantage to being able to fly your planes around the world without having to stop for fuel.
What you're looking at is a KC-135. It's cargo consists almost entirely of fuel that it can deliver to other aircraft equipped with a special receptacle for the refueling boom, seen in the lowered position below. I wish I didn't have to stop my car to get gas.
You needn't make the sexual jokes I'm sure have popped into your head by now. I've heard them all.
Have you seen the episode of Dirty Jobs where Mike is in one of these planes? He climbs into a little room at the back of the plane and looks out the window over the boom down to the plane getting refueled. Where do I sign up for that ride? No, not with the recruiter, I only want to do it once. OK, maybe twice, but not for a six year hitch. Oh, wait, there I am, in that same window. Well, at least my reflection is in that window.
In order for the boom to remain stable while extended, it's got it's own wings. This is the view of those wings from underneath the tail of the plane. I thought it made a neat picture. 6 AMW stands for the 6th Air Mobility Wing, which is stationed at Macdill. 927 ARW, that's the 927th Air Refueling Wing, which as I understand it, is part of the reserves, but shares the same planes and equipment as the 6th AMW.