From the (super short) Appscope listing:
A spirit level in web app form.
The URL to tap on or type in is spiritlevel.app .
Of course, you can turn this into an 'app' yourself with PAWA or just pin it 'as is' on your Start screen. Or just add it as a 'Favourite'. It's up to you. If you're confused by the different PWA options then check out my feature here.
Here's level PWA in action:
It's remarkably accurate - who'd have thought we'd be running utilities like these in a browser window one day? Note that the 'Clear' control toggles to a 'Zero' one and can be used if, for any reason, you need to recalibrate the level from a known surface or if your phone sensors are slightly 'out'. Unlikely, but worth knowing there's a way to reset things!