Typically, you want to start the tank with live rock, sand, and salt water, then add fish, corals, etc. after the nitrogen cycle gets established. If you already have fish in the tank, it would be a good idea to move them to a temporary home for a few days when you add the live rock.
Live rock can be considered "cured" or "uncured". Uncured rock is recently removed from the wild. It still contains bits and pieces of the animals and plants that were attached to it in the ocean. Some of those things will not be able to survive in your aquarium. Their bodies will rot in the tank, which will cause an ammonia spike.
Cured rock has been out of the ocean for a while, often in a holding tank at the Local Fish Store (LFS). It may still contain a few things that will die in your tank, but it won't produce as much ammonia as uncured rock. Fish will often survive the addition of cured live rock, but it's still a gamble.