The focus of HDMI 2.1 is on higher video bandwidth; it supports 48 Gbps with a new backwards-compatible ultra high speed HDMI cable. It also supports faster refresh rates for high video resolution -- 60 Hz for 8K and 120 Hz for 4K. The standard also supports Dynamic HDR and resolutions up to 10K for commercial and specialty use.
This new version of the HDMI specification also introduces an enhanced refresh rate that gamers will appreciate. VRR, or Variable Refresh Rate, reduces, or in some cases eliminates, lag for smoother gameplay, while Quick Frame Transport (QFT) reduces latency. Quick Media Switching, or QMS, reduces the amount of blank-screen wait time while switching media. HDMI 2.1 also includes Auto Low Latency Mode (ALLM), which automatically sets the ideal latency for the smoothest viewing experience.
If you're not sure what this HDMI upgrade means, this handy chart provided by the HDMI forum makes it clearer. You can clearly see how upgraded specifications have increased support for different features as specifications improved.