This will allow several new features, such as 4Kp120 and 8Kp60.
It also adds support for dynamic metadata HDR (like Dolby Vision HDR and HDR10+).
Here's a nice table showing the feature differences between the various versions of HDMI.
Expect new devices to support the 2.1 spec sometime in Q4 2018.
List of supported Formats:
There are lots more interesting features, so here's the official press release from the HDMI Forum. You can also read all about it on AVSForum here.