When Selesnya Sanctuary enters the battlefield, it, Zendikar's Roil, and Kodama of the East Tree trigger.
Resolve the Kodama of the East Tree trigger, choosing not to put anything from your hand onto the battlefield.
Resolve the Selesnya Sanctuary trigger, returning it from the battlefield to your hand.
Resolve the Zendikar's Roil trigger, creating a token.
When the token enters the battlefield, Kodama triggers, putting Selesnya Sanctuary from your hand onto the battlefield.
When Selesnya Sanctuary enters the battlefield, it and Zendikar's Roil trigger.
Repeat from step 4.
This clever loop thrives on the graceful interplay between your lands and triggered abilities, using Kodama of the East Tree as the engine that seamlessly translates enters-the-battlefield events into a continuous cycle. When you drop Selesnya Sanctuary, its bounce-back nature provides a repeatable way to keep feeding your hand, while Zendikar's Roil watches every land arrival to churn out a fresh 2/2 green Elemental creature token. Every time that token materializes, Kodama of the East Tree sees the new permanent and uses its triggered ability to instantly slide Selesnya Sanctuary right back onto the battlefield from your hand, restarting the entire chain for infinite tokens and landfall triggers.