It’s a thin, persistent bar on the left side of the page with tools and shortcuts. By default, it’s where you find bookmarks, history, downloads and things like that, but you can also add custom websites to it as well. They pop out either over or alongside the main window (depending on whether the pin button is pressed), and they display the mobile webpage when available, to be more usable in such a small window. It’s how I use Discord and Mastodon.