The short explanation
The pixel clock is how fast pixels leave the source. More pixels, or more frames, means a higher clock. 300MHz covers 4K30 and 4K60 4:2:0; 600MHz is needed for 4K60 4:4:4; 1500MHz is the HDMI 2.1 ceiling.
Gbps and MHz describe the same limit from different sides: 600MHz corresponds to 18Gbps, 1500MHz to 48Gbps.
Where you run into it
Cables and devices are often rated in MHz, so the number tells you directly which formats are on the table. A cable sold as 300MHz will not carry 4K60 4:4:4 no matter what else is in the chain.
It also explains a common frustration: a display that accepts 4K60 but refuses 4K60 4:4:4 is a 600MHz problem, and on many televisions it is a per-input setting rather than a hardware limit.
How the DR HDMI 8K handles it
The DR HDMI 8K passes 1500MHz / 48Gbps and reports the live TMDS clock and ratio. Its EDID library is organized partly by clock — 600MHz tables for 4K60 and 4K50, 300MHz tables for 4K30 and 4K60 4:2:0 — so you can advertise a specific ceiling and see whether the chain behaves better below it.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.