In plain language
High Bit Rate audio is a transport mode carrying far more data than the standard compressed streams — roughly eight times the capacity, which is what lossless multichannel needs.
Why it matters in your setup
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.
What the DR HDMI 8K does with 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.
Need the hardware?
Open the DR HDMI 8K product page on HDfury.uk.