Ivy is creepy. It creeps everywhere. As it does so, it extends roots into or onto whatever it creeps over. Not only does it do this to climb, but it also does this to propagate. It literally roots as it goes, to function more as a swarm of countless small plants instead of a single big plant. A bit of well rooted stem may grow independently from the original as a ‘layer’.
Many vines do the same, even if they are normally climbing vines that just happen to fall onto the ground. A few shrubs and trees, especially riparian sorts, are happy to do it also if lower limbs lay onto damp soil. Such a rooted stem is known as a layer because it lays on the ground to root. In a home garden, a layer might root below a layer of mulch or soil.
In fact, many plants in home gardens are easy to propagate by intentional layering. Many develop roots more reliably if layered than if propagated from cuttings. They grow almost like cuttings, but while attached to the original plants to sustain them. Layering produces only one or a few new plants, unlike cuttings, but for most home gardens, that is enough.
Some plants are easier to layer than others. Pines and most eucalypti are uncooperative to the technique. Rhododendrons, azaleas and camellias layer relatively easily, but may take more than a year to finish. Elms and magnolias are even happier to layer, but rarely retain low stems. Some plants layer best after spring bloom. Others layer through winter.
An intentionally layered stem should be partially buried, with a few inches of stem below the soil, and a few inches of the tip of the stem protruding above the soil. Notching about a third of the way through the underside and applying rooting hormone promotes rooting. Regular watering is necessary through the process, which continues at least until winter.