HDMI 2.1 is no small iterative change over 2.0
HDMI has been around for years, but that doesn't mean it's still the same interface it was 15 years ago. The HDMI digital video/audio interface was officially released in 2002, but the ubiquitous standard has undergone a number of revisions since then. The most recent of these is HDMI 2.1, revealed January 4 during CES. Though it's called version 2.1 (as opposed to, say, 3.0), it's got some serious advantages over HDMI 2.0—released over three years ago.
The first major upgrade is the bandwidth the HDMI 2.1 interface is capable of. HDMI 2.1 will support up to 48 Gbps bandwidth. To put that in perspective, that's more than twice as much as HDMI 2.0 (18 Gbps) and over four times more than HDMI 1.4 (10.2 Gbps). This added bandwidth will allow future-proofing for resolutions and refresh rates not even possible with today's high-end PCs: 4K/120Hz, 5K/120Hz, 8K/120Hz, and an incredible 10K/120Hz are just a few possibilities.
Beyond resolutions and refresh rates, 2.1 will support variable refresh rate, a frame-buffering method similar to AMD's FreeSync and Nvidia's G-Sync, which allows for smoother gameplay and lower input lag. The best part is you won't need a new cable for this—unless 4k/60Hz is simply too poor of an experience for you. HDMI 2.1 also supports an improved implementation of HDR. In 2.0, the instructions for how the display should handle HDR are applied to an entire program. HDMI 2.1 will allow per-frame HDR image adjustment.
There are certainly more things that HDMI 2.1 will be able to do but are yet to be announced. HDMI's founders will announce the full specifications in Q2 2017.