When your opponent offers you the doubling cube, you must take (accept) or pass (decline) the cube before the game can continue.
Click the Take button to accept the cube and continue the game. You now own the cube and the match will be played for the new cube value.
Click the Pass button to decline the cube offer. The current game ends and you lose the number of points on the cube before it was offered. If the match is not over, the next game will begin.
If your match options allow redoubles and you can redouble, you will see a Redouble button in addition to the Take and Pass buttons. Click this button to take the cube and immediately offer it back to your opponent.