What do laptop processor letters like U, H and HX mean?

They are suffixes indicating the power and thermal class of the chip, and they matter considerably more than the model number for how a laptop actually performs.

Intel's laptop suffixes, roughly by power:

U — ultra-low power, typically around 15W base. Thin, light, fanless or near-silent machines with long battery life. Fine for office work, browsing and video; limited for sustained heavy work.

P — a middle tier introduced more recently, around 28W. More performance in a still-portable chassis.

H — high performance, around 45W. Thicker laptops with real cooling. Suited to development, content creation and gaming.

HX — the highest tier, effectively desktop silicon in a laptop, 55W and above. Workstation and high-end gaming machines, with substantial cooling and short battery life.

Y — historically the lowest power, largely superseded.

AMD uses its own scheme, with U for low power, HS for a middle tier and HX for the highest, plus a numbering system where individual digits encode year, tier and architecture.

Why the suffix matters more than the number. A higher-numbered U-series chip will frequently be slower under sustained load than a lower-numbered H-series one, because it is thermally limited. Comparing model numbers across suffixes is misleading, and this is the most common purchasing error.

The point most specifications hide: the chassis matters as much as the chip. Two laptops with the identical processor can perform very differently, because performance under sustained load depends on how much heat the cooling system can remove. A thin machine with a small fan will throttle a powerful chip within minutes, delivering far less than the specification implies.

This is why reviews measure sustained rather than peak performance, and why a well-cooled mid-tier machine can beat a poorly cooled high-tier one.

Also check: core counts, and whether efficiency and performance cores are mixed, which affects how workloads are scheduled.