What is this? and maybe more important, why?
Basically, it's a URL for the surface of the earth.
According to Fréderic Filloux, "Addressing 4 billion People in Three Words", Monday Note 11/30/2015:
Last week in New York, at the Next Billion conference organized by Quartz, Chris Sheldrick, the CEO of What3Words, captured his audience with strong arguments: 75% of the earth population, i.e. four billion people, “don’t exist” because they have no physical address. This cohort of “unaddressed” can’t open a bank account, can’t deal properly with an hospital or an administration, let alone get a delivery. This is a major impediment to global development. […]
In his previous job, Chris Sheldrick (now 33) had his epiphany when organizing large musical events around the world. Tons of material had to be shipped at a specific location and date/time. After several mishaps, he too tried using GPS coordinates to make dozens of flight cases converge at the right time and place. But people got confused with lat/long, sometimes mixing ones and sevens, etc. After a dramatic mistake that almost ruined a large wedding party in the Italian countryside, he vented his frustration to a mathematician friend who then suggested the following: why not replacing GPS coordinates with actual words that anyone can understand and memorize? Sheldrick’s mathematician pal came up with a simple idea: a combination of three words, in any language, could specify every 3 meters by 3 meters square in the world. More than enough to designate a hut in Siberia or a building doorway in Tokyo. Altogether, 40,000 words combined in triplets label 57 trillion squares.
Checking the math: The total surface area of the earth is 4*pi*6371^2 = 510064472 square kilometers, or about 5.101e+14 square meters. So we need about 5.101e+14/9 = 5.67e+13 labels for squares three meters on a side covering the earth's surface. The number of 3-tuples formed from 40,000 words is 40000^3 = 6.4e+13, which is about 7 trillion more than we need.
Rather than use a conventional approach to error detection, which would use a much larger space of possible word combinations, most of which would be invalid, they do this:
The what3words algorithm actively shuffles similar-sounding 3 word combinations around the world to enable both human and automated intelligent error-checking (e.g. table.chair.lamp & table.chair.lamps are on different continents).
If you enter a 3 word address slightly incorrectly and the result is still a valid what3words result, the location will be so far away from your intended area that it will be immediately obvious to the person searching or an intelligent automated error-detection system.
So far, no one has noticed the possibilities for a new form of geo-aleatoric poetry.
Update — Gile Rhys Jones points out that I'm wrong — many people have noticed the poetry possibilities. There's the official what3words poetry competition, a tumblr page, a blog post by Darren Wiens, …