It's a single, self-contained HTML file. You download it, add data, then overwrite the file with a new version which includes changes.
If you want to go fancy, you can install it on a server with Node.js and stuff, but there's not a need to go there.
These days you might run across some hiccups in getting it to work in your browser of choice. Over the years the browser vendors became less and less inclined to allow javascript and extensions attain local file editing permissions. That's pretty reasonable, but annoying.
A pretty decent alternative that I've been using for months, now, is an app called Tiddly Desktop. It's an NW.js-based app so it has no trouble accessing the local filesystems.
Never experienced any problems whatsoever across FF/Chrome/etc
I just generate a new HTML file with changes for saving.
Requires remembering to do something to save changes, but I come from the time where autosaving wasn't the norm anyway :)