Every spot in the United States (lower 48, Alaska, Hawaii) is cut into squares about 10 feet across, 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; nothing is sent anywhere until you tap the Google Maps button.

Words only mean something to R3W tools. They are not what3words. Reverse lookup (pick a place on a map, get its words) is planned.