The short explanation
High Bit Rate audio is a transport mode carrying far more data than the standard compressed streams — many times the capacity, which is what lossless multichannel needs.
Where you run into it
It is the dividing line between the formats optical can carry and the ones it cannot. Dolby Digital and DTS fit down an optical cable; TrueHD and DTS-HD MA do not, and never will.
It also matters for ARC. Plain ARC has no room for HBR; eARC does, which is the technical reason eARC arrived at all.
How the DR HDMI 8K handles it
HBR appears in the DR HDMI 8K's supported audio list alongside PCM up to 192kHz, compressed audio, DSD, DST, DTS, DTS-HD, TrueHD and DTS-MA. The audio flags let you advertise or remove the lossless formats individually, which is how you test whether a missing TrueHD track is a capability problem.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.