A hanging mount is a vertical surface (usually wood), with a hook, which allows you to hang vining and epiphytic plant species.
Mounting plants in this way allows them to grow as they would in nature (climbing, draping, and so on, based on the species).
There are many species which prefer to grow epiphytically, including a good number of orchids and ferns.
Typically, the mount will have a sphagnum moss substrate.
These mounts are usually used in grow tanks and greenhouses.
Hanging mounts also make more efficient use of space, because you can have them hanging above potted plants.