Can anyone ‘splain to me what is loose nuclear material?

Like, whoops this stuff just fell out of my pants…

Obama and Biden will secure all loose nuclear materials in the world within four years. While working to secure existing stockpiles of nuclear material, Obama and Biden will negotiate a verifiable global ban on the production of new nuclear weapons material. This will deny terrorists the ability to steal or buy loose nuclear materials.

From Obama’s Foreign Policy page on whitehouse.gov