This is the achievement related to completing the game. You must complete all levels of the first 5 Chapters of the game. Completing levels only requires that you get to the exit portal. You don't have to collect any lights (except the required amount to unlock subsequent chapters). Once you have done this, a rose icon will appear within the Chapter 5 level select screen. Click on it and complete this very simple final level (just get to the portal, there are no lights to collect). The achievement unlocks and then the credits roll. Thanks to PlexShaw for the tip.