HDMI 2.1 adds support for 8K@60Hz and 4K@120Hz, Dynamic HDR, increased bandwidth using new 48G cabling for 48Gbps bandwidth, and remains backwards-compatible with existing versions of the HDMI specification. Of Dynamic HDR for HDMI, "ensures every moment of a video is displayed at its ideal values for depth, detail, brightness, contrast, and wider color gamuts—on a scene-by-scene or even a frame-by-frame basis."
HDMI 2.1 also features a mode similar to FreeSync / AdaptiveSync, which in the HDMI space is being called Game Mode VRR. VRR being for the Variable Refresh Rate. The announcement today explains, "Game Mode VRR features variable refresh rate, which enables a 3D graphics processor to display the image at the moment it is rendered for more fluid and better detailed gameplay, and for reducing or eliminating lag, stutter, and frame tearing."
HDMI 2.1 will be officially released in Q2'2017. More details via today's press release.