What is the Relay app, exactly?

It's a desktop companion for your Supernote. It mirrors the To-Do area of your Supernote (the same to-dos and lists you see in the Supernote Partner app and on your device), keeps them two-way in sync with your Supernote account and device, and can hand those to-dos off to task apps you already use — Todoist, Apple Reminders, Things 3, TickTick, Google Tasks, Asana and more. In short: it brings your Supernote to-dos onto your Mac and lets you push them out to other apps.