Shuttle Discovery is rolling out for the penultimate space shuttle mission. The 140th and almost last (planned) shuttle launch.
They are rolling out after fixing a problem with a loose nut. During assembly with the external tank, a nut came off.
Now, we've all been working on a computer or appliance and dropped a screw. You might curse as you hear it rattle its way down past all the sharp exposed metal edges. You consider fishing around and tearing up your fingers. Then you just flip the darn thing over and shake until the screw rattles its way back out.
You can't do that if you drop a nut inside the shuttle. (Well, technically, you probably could. Use the vehicle assembly hoist and lift the shuttle upside down by its mount points. But it's against regulations. And the last guy they caught doing that got soooooo fired.)
So instead they opened the aft compartment, retrieved the piece, and put it back into position. Now when you consider how big that machine is and how many fittings there are and the fact that they noticed and had to retrieve a single nut... It's understandable why launching one of these things takes a lot of work. This is rocket science.
Best wishes to Discovery for a great mission!