It's rock that's live... duh!
Just means the rock has been in an established aquarium or directly from the ocean and is loaded with beneficial bacteria (along with a host of hitchhikers usually)
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry