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 map tiles come from OpenStreetMap; 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.