These are two methods of propagating Rhododendrons.
Layering is when a low lying stem is bent to the ground, anchored down into the soil so that roots grow from the anchor point. The tip can then be cut off and grown as a new plant. I don't know that this is common practice with Rhododendrons.
Much more common is grafting. Here, the young stem of an easy from seed Rhododenron is used as a root stock, it's tip is cut off and the tip of a mature but more difficult R. is bound onto it in such a way that that the plant grows in the manner of the tip plant. The trouble with this is that after many years the root section can become dominant and you lose the more valuable top species. The Victorians used R. ponticum as a root stock for some of their exquisit plants but now the purple ponticum has become an invasive weed in some locations.
Most of the Garden Center Rhododendrons you buy will be grafted.