When you don’t have a domino (aka ‘bone’) in hand that fits on to any of the trains on the board, the game will ask you to draw one. When you press ‘Draw’ a new domino will appear face down in the ‘boneyard’. The boneyard is a shoe on which the ‘Draw’ button is located. In order to use the new domino, you need to drag it out of the shoe / boneyard and place it either on the board or the shelf. The domino is turned face up automatically when it is moved.

Note, if the domino is left in the shoe / boneyard, the game will message you accordingly.