I don't understand what you do not get. In the link you sent they claim to be a privacy oriented web browser based on chromium

It's a browser (chromium based) with a really nice UI that people love, I am intrigued but haven't used it because I find the requirement to create an account off-putting.

The “what makes Arc different from other browsers” section is particularly funny.

> Arc is to your ex-browser what the iPhone was to cellphones. Or as one of our members said “like moving from a PC to a Mac.” It’s from the future — and just feels great.

As a person that recently started using it: it has something like "tree style tabs", and sort of a hybrid merge of the concepts of tabs and bookmarks. In other words, the tabs work more like files on disk -- open/closed, sorted into folders. I'm probably not explaining it well either, but I encourage you to try it if you ever wanted to experiment with alternative tab management (tree style tab, tab groups etc). It's a concept that clicked for me quickly once I started using it, and now I'm angry since I want to use Firefox for philosophical reasons but don't want to go back to regular tabs.