This is a stupid simple multi-device reminders application. The idea is that you can use a mobile automation (Tasker...) or other entrypoint post a reminder, and it can then show up persistently on any of your device(s) that you configured, until it is dismissed. Great for quickly making a note of something you'll have to deal with when you're back at your computer.
I have this on a VPS fronted by Caddy using a private CA. Notably there is no authentication. It turns out you can make security by obscurity pretty darn good by simply having a reverse proxy reject requests that don't come with the appropriate SNI, and use a private CA for the TLS, because then the IP address / domain name pair basically acts like a username and password if they are not published elsewhere. Besides, what are you going to do? Hack the most recent 15 minutes of my todo list? Lol.