As this game is an endless runner the inevitable will eventually happen and your dragon will fall into 'Starving' status, then 'Critical', and finally die. Once this has happened a message box will appear on the screen with the message 'Save Me' along with the option to watch ads or pay gems to continue.
It is more beneficial to save your gems for buying purple eggs or costumes and avoid the temptation of spending them to continue a game from where you left off, unless of course you have an excess amount of gems. You should instead watch the advertisement video when it is presented to you to continue your game with a full bar of health and two lives.