What does "Sync" actually do, and what's getting synced?

"Sync" keeps your to-dos in step between this app and Supernote. Anything in your Supernote to-do app flows into the app, and any change you make in the app flows back. Both directions go through the Supernote cloud — the same place your device syncs to — so a to-do you tick off on your tablet shows as done here, and a to-do you add here appears on your tablet after its next cloud sync. Sync is only about to-dos and lists; it's different from "Push", which sends to-dos out to other apps.