Sky Pillar is a daily game mode that is unlocked at lvl 60 together with DNA Skills. In this game mode you complete several battles across the floors until you reach Floor 200, wich is the highest floor at the moment.
This game mode will reward you with materials for the Gene / DNA Skills of your pokémon and also with Overlord Charizard X Stickers.
Every floor that you complete can be raided. So once you reach floor 200, your 5 daily battle chances can be used to just raid the floors.
To learn more about DNA Skills and where to use the resources that you obtain from Sky Pillar, check this article - DNA Skills.