Seems pretty straightforward. Download the app, buy some data for the country/countries that you will be visiting, and then the app installs an eSIM for you. Got to do this before you arrive because it requires data. Then while there, monitor the data usage and top up as needed. It looks like depending on the location, the plans start as low as $4 for 1GB (with data getting cheaper in the bigger plans- 10 GB for $16 in most of Europe).

I think I’m going to try this on my trip to Europe in December.