Jot does not yet have a mobile app, but on most phones you can save the browser version to the home screen and it will function similarly to an app.
On iPhone, open your browser and look for a menu icon. On Safari it's the icon at the bottom. Choose "Add to home screen" from the menu.
On Android, open your browser and look for a menu icon. On Chrome it's usually at the top, but it might be different for you. Choose "Add to Home Screen" from the menu.