In plain language
Under the older TMDS scheme there are three data lanes plus a separate clock lane. HDMI 2.1's FRL repurposes the clock lane as a fourth data lane and embeds the clock in the data, which is where a large part of the extra bandwidth comes from.
A readout such as 4 lanes / 40Gbps means four lanes at 10Gbps each.
Why it matters in your setup
The lane count tells you which scheme the link is really using. Three lanes means TMDS; four means FRL. If you expected 4K120 and see three lanes, the two ends never made the jump — and that is an EDID or device question rather than a cable one.
All four lanes also have to be electrically sound. A cable with one marginal conductor can pass at a lower rate and fail at a higher one, which is exactly the intermittent behaviour that gets blamed on everything else.
What the DR HDMI 8K does with it
The DR HDMI 8K reports the FRL rate, the number of lanes and the per-lane bandwidth on both the OLED and the web interface. That single line is usually the fastest answer to “did my chain actually reach HDMI 2.1 speeds”.
Need the hardware?
Open the DR HDMI 8K product page on HDfury.uk.