A fiber optic adapter or coupler is a special connector designed to mate or connect two ends of a fiber optic cable with high precision. The adapter uses a simple design: the ends of two separate fiber optic cables with fiber optic connectors fit into two slots opposite each other. The slots are designed to precisely align the two ends so that there is minimal to no signal loss.
Techopedia explains Fiber Optic Adapter
Fiber optic adapters mate two ends of different fibers; hence they are also called couplers. There are versions that connect single fibers together (simplex); others connect two fibers for duplex mode, and some even connect up to four fibers.
Precision mating is done through the alignment sleeves, which are made of either ceramic or metal such as copper in order to provide rigid support for the connectors, whereas the body of the connector itself is made of plastic. There are fiber optic adapters/couplers made for every type of fiber optic connector available.