What is the small symbol in the bottom right corner of photos?

You'll run across a few others as you go along. Apple refer to them as badges. You can find a list in the User Manual, in section "Understanding Badge Overlays" of the chapter "Working with Metadata".
http://documentation.apple.com/en/aperture/usermanual/#chapter=11%26section=9

Hope this helps too.

I'm trying to move pictures into a folder. There's a symbol that looks like a grenade in the bottom right and any pictures with this symbol cannot be moved into my folder. They are already in aperture but why can't I move them?

Picture can only be contained in either albums or projects, not folders. Projects and albums on the other hand can be in folders. The grenade you're referring to means you image has metadata applied to it. But this has nothing to do with the moving process.

Welcome to the forum.

That means that its a referenced image, with the original image file stored outside the Library.

Ian