In the game there are a grid of gems, you have to destroy the gems by swapping them. The gems will be destroyed if a line of 3 or more gems of the same kind are there. When some gems are destroyed, new gems will be created. You need to destroy gems as quickly as possible in order to advance to the next level.
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.