Every spot in the United States (lower 48, Alaska, Hawaii) is cut into squares about 9 by 10 feet (the same squares Google's Plus Codes use), and each square has its own three words from a fixed list of 16,384 English words. Neighbouring squares get unrelated words, so a mistyped word usually lands somewhere obviously wrong, or is rejected. Everything runs in this page. The only outside requests go to OpenStreetMap: map tiles, and a town-name search if you paste a short Plus Code with its town. Your words and positions are never sent to any server.

Words only mean something to R3W tools. They are not what3words. R3W squares are exactly Google's Plus Code squares, so a Plus Code and its three words always name the same spot.