Not just a normal Mahjong game where you need to combine 2 of the same stones... This one is more difficult, you now need to add up the values on the stone so they are together exact 10.
Click on 2 of the same stones and make sure the value of the stones together is exact 10. So combine a 7 with a 3 and a 2 with an 8.
A context menu should be shown when right-clicking or, on touch devices, long-pressing in the game. This menu especially allows to play in full screen and control the volume.