It can be used where you have a long stem that just will not bend to the ground. The stem is wounded and then sphagnum moss (damp) is wrapped around the area before being covered with a sheet of clear - usually - durable plastic and fastened top and bottom with tape.

Roots form and grow out into the sphagnum.

When they've developed enough then the branch can be cut off below the new root zone, plastic off, and potted up. Then kept in a sheltered place for a month or so.

Can be used for some leggy house plants, bonsai work, trees/shrubs.

Sort of like striking cuttings in water - except they're still on the parent plant.

Just as with layering - you may need to be patient with some species.