Root tabs are fertiliser in the form of a tablet which is pushed into the substrate near the plant roots.
Java fern has a rhizome. It's the thick stem that the leaves are attached to. The roots are the thin hair like things that are attached to the other side of the rhizome. The rhizome will rot if covered, but the roots are OK in the substrate. Java fern is usually attached to a rock, piece of wood, plastic 'dead tree trunk' etc to stop the rhizome getting accidentally covered.
Java moss is usually grown on something. Mosses don't have proper roots.