HDMI 2.0 ouputs and their support for 3840x2160 resolution at 60 Hz are still not ubiquitous even on brand-new PCs. As an example, the UHD Graphics unit included in Intel's latest eighth-generation Core processors needs help from a converter chip to achieve that resolution and refresh rate. The HDMI Forum is nevertheless charging ahead and has released the HDMI 2.1 specification. The new spec includes support for resolutions up to 10K (10240x4320) at refresh rates up to 120 Hz. The mandatory Ultra High Speed HDMI cable can now carry up to 48 Gbps and allows for Display Strem Compression (DSC).
Among other features, HDMI 2.1 also supports variable refresh rates, an automatic low-latency mode, and HDR metadata. HDR support is extended from a using a single image descriptor for an entire piece of content to a new Dynamic HDR mode that can use a different image descriptor optimized on a screen-by-screen or even frame-by-frame basis. The new standard further supports next-gen color spaces like BT.2020 for future HD content.
The new Ultra High Speed HDMI cable uses the same Type A, C, and D connectors as its forebears, but offers increased 48 Gbps bandwidth compared to the 18 Gbps possible with existing HDMI 2.0 cables. The new Enhanced Audio Return Channel (eARC) is backwards-compatible with the existing ARC and has support for uncompressed and object-based audio formats like DTS:X and Dolby Atmos.
Much like FreeSync and G-Sync, the variable refresh rate tech baked into HDMI 2.1 should help decrease input latency and eliminate frame tearing. The new Quick Media Switching feature allows changes in refresh rates and resolution without the customary and annoying display blackout. The new feature list also includes Quick Frame Transport, a reduced-latency mode for games, mixed reality, and karaoke. An Auto Low Latency mode allows source units to establish the ideal latency settings for different types of media, too.
If you're looking for more information, check out the HDMI Forum's Release Presentation. The document touches on the spec's main points without getting too bogged down in implementation details.