The goal of Mahjong is to remove all tiles from the board by combining pairs. The tiles with identical patterns can be selected if they are not covered by another tile and if they have one of their big side free.
Use your mouse to select and click on two identical tiles to remove.
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.