This is Indexer keeps track of various actions for a Schema:

• inserts: Incremented for each insert or upsert
• deletes: Incremented for each delete
• collection: Incremented for each insert, update, or delete
• updates: Incremented for each update (via Repo.update/*)
• changes: Incremented for each insert, upsert, delete, or update

These keys are useful for creating watches that will notify your application of those actions. For example, if you create a watch on the inserts key, your application will be notified when a new record is inserted, and you can react however you like.

See it in action: Sync Engine Part II - Collections