I'm pretty sure that's a special mode you enter to assemble/disassemble structures.
I was rather impressed at this feature. I always did complain about people leaving junk lying around in the hubs in previous games. You guys have been living in New Vegas for how long? And you can't even manage to move a few broken down cars, or some bricks? Come on, make the place look good at least. Now I can do just that. I'm about 75% excited at cleaning stuff up, and 25% excited about rebuilding stuff.
I wonder if there are specific foundations you have to build on, or if you can expand foundations, or if you can put it anywhere and everywhere within the building zones. I've also wondered if you can assign some buildings to specific individuals and make it their 'home'.
But yeah, other than customizing my guns, this is probably my favorite feature I've seen thus far.