Definition:
Ayni is the fundamental principle of reciprocity in Andean cosmology, meaning "sacred exchange" or "balanced giving and receiving." 

In Practice:
Ayni governs all relationships in Andean spirituality—between humans and nature, the living and the dead, the physical and spiritual realms. Every offering requires a return. Every blessing creates an obligation. When you ask the earth for something, you must give something back. When ancestors help you, you must feed them. Ayni is the cosmic law that maintains balance across all worlds. 

Example:
If you receive a successful harvest, you offer back to Pachamama. If an ancestor helps you in business, you light candles and pour libations for them. 

Source:
Traditional Andean/Ashaninka practice as taught by Karyna's grandmother and great-grandmother, TheSyncretistRealm.com