When a tile is placed on the board they will light up with either a yellow or green outline.

A yellow outline means that the word on the board cannot be played, either because it is not a valid word or because it is not connected to any tiles that have already been placed on the board. Too many connections may also cause a word to not be approved, since the new letter has to form a valid meaning in all places it's connecting to.

A green outline means that the word is valid and can be played by tapping 'Submit'.