The rhizome is the base of the plant that all of the leaves are attached to.
In the past, with Anubias and African ferns I have temporarily buried the rhizome in the substrate for the first two or three months until roots have developed and taken hold. I would then gently pull the rhizome up through the gravel until it was above the surface of the substrate for the remaining growth of the plant. I never had any problems with plants rotting away.
Now what I do is attach the plants to rocks with rubber bands. After a couple of months the rubber bands will have degraded to the point where they break and can be removed while the plants are attached where I wanted them.