An Ultra High Speed HDMI cable is the HDMI Forum’s certified cable category for HDMI 2.1 (and later) links that need up to 48 Gbit/s of bandwidth. It is tested to carry the highest HDMI 2.1 video rates—including formats such as 4K at 120 Hz and 8K at 60 Hz—plus features such as HDR, Variable Refresh Rate (VRR), and Enhanced Audio Return Channel (eARC). Lower cable grades (Standard, High Speed, Premium High Speed) are not guaranteed for that full 48 Gbit/s class.
Certified Ultra High Speed cables must pass Forum Authorized Testing Center checks (including low EMI) for each length, and packaging should show the official Ultra High Speed HDMI Certification Label so buyers can verify compliance. Older HDMI devices still work with these cables; the ultra-high-speed rating mainly matters when source and display both need the higher bandwidth.
Source: HDMI — Wikipedia (cable certifications table); also HDMI Forum Ultra High Speed HDMI Cable.