This feature lets you set the rel attribute of a link on your site.
There’s a few use-cases for using the rel attribute, some people set links to 3rd party sites as rel=nofollow to signal to Google not to consider the link as part of the ranking algorithm, other uses are for things like user generated content (ugc) or sponsored. Here’s some info on using it:
I’m not sure what Webflow does/doesn’t allow (it should be pretty easy to test though - let us know what you find out if you test it). You can’t add any attribute, but you may be able to apply any value to the rel attribute