I can imagine a lot of ways how this might work, but the website does not explain much. Maybe it should have a short screencast as an example. Or at least screenshots. I also assume this collects a lot of data on the user? How is that handled?
Yes, a screencast would be nice, but more than that, there's no Privacy Policy? Edit: there is, but they don't link to it on the homepage https://jointoucan.com/privacy-policy
If this is the company's approach on handling very private user data (ie. the contents of a number of websites they visit, including facebook, airbnb, office365, all of google.com), then this is a hard pass from me, and a huge red flag.
EDIT: here's a list of all sites the extension accesses: https://paste.q3k.org/paste/uj-GbID4#g7IYwrXiF6zXlnlxQYguMlr...
EDIT2: looking at the source code of the extension (extracted from the source map), it at least tracks and sends off any tab hostname (or URL?) you've visited: https://paste.q3k.org/paste/gxklokkF#055mmZ9Qu-zeiAwXGpWqSpw... , for any of the URLs in the allowed URLs list (which in turn has some websites whitelisted that you really might not want others to know you have visited)
EDIT3: if I read the code correctly, they actually send off the entire URL, not just the hostname. But someone would have to check this in practice to be sure.