FRL replaces TMDS
Earlier HDMI versions send video over three data lanes plus a separate clock lane, called TMDS, at up to 6 Gbps per lane in HDMI 2.0. That's a headline 18 Gbps, and 14.4 Gbps of data after the 8b/10b coding. HDMI 2.1 adds a second mode, Fixed Rate Link (FRL). It uses four data lanes with the clock built into the data, at up to 12 Gbps per lane, and a more efficient 16b/18b coding. The headline is 48 Gbps, and 42.6 Gbps of that is data.
FRL isn't one speed. A link can run 3 lanes at 3 or 6 Gbps, or 4 lanes at 6, 8, 10 or 12 Gbps, and the source and display use a rate that both of them and the cable support. That gives totals of 9, 18, 24, 32, 40 and 48 Gbps.
What fits on it
With 42.6 Gbps of data, HDMI 2.1 carries 4K at 120 Hz in 10-bit full color, which needs 35.6 Gbps with CTA-861 timing. Go up to 4K at 240 Hz and the mode needs 68.6 Gbps, so it only works with Display Stream Compression.
8K at 60 Hz in 10-bit needs 62.1 Gbps, which also takes compression. The 8K 60 Hz page goes through the chroma and compression options.
Features and the version trap
HDMI 2.1 is also a feature list: variable refresh rate (VRR), Quick Media Switching (QMS), Quick Frame Transport (QFT), Auto Low Latency Mode (ALLM), eARC and dynamic HDR. Devices aren't required to support the maximum bandwidth of the version they implement, and they don't have to support every feature either. A port labeled HDMI 2.1 can stop at a lower FRL rate, so the version number tells you less than the list of modes the maker publishes.
HDMI 2.1 was released in November 2017. HDMI 2.1a followed in February 2022 and added Source-Based Tone Mapping, and 2.1b followed in August 2023.
The cable matters
The full 48 Gbps works only with a cable certified for it, called Ultra High Speed HDMI. Certification is mandatory and done at an HDMI Forum Authorized Testing Center, and a certified cable carries a label with a QR code that you can scan to check it. These cables arrived in 2020. If a high refresh rate mode won't turn on, the cable is the first thing to check, then the port's own limit.