This tab isn’t new, but it has had a bit of a redesign, and some new features have been added. They include HDR (for keeping very dark and very bright areas of a picture visible — unfortunately, it’s for Google One users only) and White point (for adjusting the white balance in the photo). Again, it adds to the range of edits you can make right in your browser.
For example, you can use Highlights to reduce the brightness of the lighter areas in the image or Shadow to brighten up the darker spots.