cured rock is rock that already had its "die off". it's rock that is pulled out of the ocean, transported, put in water for a couple weeks, scrubbed of all dead stuff. it can be done in the lfs or in your garage/basement/non stocked tank.
the die off causes ammonia cycle to begin, which is why it's dangerous to trust that the rock is cured if you have fish already. most people will add a little at a time to a stocked tank, or all at once to a tank you are just starting.