It's a Pseudobulb epiphyte, that is, a plant which storage organ is derived from the part of a stem between two leaf nodes and that grows upon another plant (generally a tree) in a non-parasitically way.
In some species, it is hardly swollen at all and looks like a normal stem with many leaves while at the other extreme, some genera such as Bulbophyllum have single, spherical pseudobulbs with one (or two) leafs at the apex of each.