It's a script that you put in place of the raw url of a bookmark in your browser. For these ones here you'd make a new bookmark and past in the script there (adjusting the fediverse url for your home community so it can correctly redirect you) and it runs some (usually) javascript that manipulates the page your on in a way to direct you to the asked for location.

Its a much more lightweight way to do a single thing if that's all you're needing and since you can see the code you can also be sure, unlike a chrome plugin, that it's not doing other weird stuff.