you will find a ton of information if you search "Curing live rock" on this board, but here's the short answer: live rock is full of bacteria, tiny critters, and living things such as algae, etc. In the time it takes to travel from the ocean to your house, some of that stuff will suffer and die off. If you put it directly into your tank, the decay of the dead or nearly dead stuff will cause an ammonia spike and may cause some trouble for your tank. So, the idea in curing is to put the rock in "isolation" alone in a tank or container of some kind until it is stable. This is a good time to examine the rock for unwanted stuff, too, like aiptasia, unwanted crabs, mantis shrimp, bristle worms (some ppl don't want them) so you can get the rock in the condition you want before you put it in your show tank.
If you buy rock from your local fish store, it is usually coming out of a tank, so its already had time to go through this process and thus can be placed right into your tank. Thats it in a nutshell.
Vendors will say their rock is "cured" but most ppl feel ANY transport of rock merits a curing process. Like I said, do a search and you will find lots of other thoughts on this. Enjoy!
