what3words is a system for identifying locations on the Earth's surface by a group of three words, each such combo corresponding to a 3-by-3 meter square. It is intended as an addressing system to replace such things as street addresses and latitude-and-longitude coordinates, and is actually being used by the postal system in Mongolia for this purpose. Its system is proprietary, and they have used takedown notices in an attempt to squelch open-sourced reverse-engineered implementations of it, causing some flack. At least one such implementation is still currently online.
The algorithm uses a list of 40,000 words and a system of dividing the world into blocks and doing mathematical calculations on their coordinates to ultimately yield three indices into that list, where nearby points have totally different words connected with them.
OpenStreetMap has disallowed adding What3Words identifiers anywhere in their system due to their proprietary nature.