What is What3Words?
What3Words is a proprietary geocode system that assigns a three-word identifier to each 3m x 3m square on Earth.
What3Words is a proprietary, non-open, patented system that is trigger-happy with litigations. If you are need a geocode system, you are highly advised against using it.
What3Words can be considered the Oracle of geocoding systems. Their legal department is considerably more active than any technical department they might have.
What3Words has the common business model of pretending to be a standard and creatively convincing governments and businesses to use their system instead of a more widespread open system. Once their system has enough mindshare, the only viable business path forwards is to take it away and charge absurd prices for it. Pay or lose access to the popular method of describing where you are.
What3Words Map API
Their website has been observed to do the following API calls during regular use.
Convert coordinates to 3-word identifier
Convert 3-word identifier to coordinates
Get grid boxes