I'm referring to actual broken rocks gathered from actual reefs in foreign countries. Like this: http://premiumaquatics.com/aquatic-supply/Liverock.html
It's really porous and filled with beneficial bacteria that will actually filter your water through the nitrogen cycle. Additionally it often brings a host of hitchhikers to your aquarium. Small Copepods (essentially aquatic bugs that fish love), star fish, worms, sponges , corals, crabs, and even more exotic surprises... Not always good though. It's also usually covered in colorful coralline algae, the hard colorful (typically blue) crust that covers most things in a tropical reef.