The waste pool is where the spent fuel rods are stored after they have been removed from the reactor. Very radiocative and toxic stuff. This is the same waste that the government wants to store in Yucca Mountain, FYI.
The problem is that unlike the reactors, which are semi-safely behind several feet of reinforced concrete, the waste pools are almost all unprotected, and therefore an attractive target.
For a little more background, you might want to check out the article I linked to in post #23.