GreenDao is an ORM library that provides high performance for CRUD operations on SQLite databases in Android apps. It uses code generation to map objects to database tables, allowing data to be accessed and queried using objects rather than raw SQL. Some key features include object mapping, query building, caching, and bulk operations. The documentation provides examples of how to set up GreenDao in a project, define entity classes, perform queries, inserts, updates and deletes on objects.