Sorry if this question is too simple for the community but I really could not find an explanation. A detailed answer would be appreciated.
HPCs work by allowing you to run jobs on many computers with many CPUs in parallel. CPUh refers to how many CPUs are being used for how long (the h in CPUh stands for hours).
For instance, you may have a job that needs 64 CPUs and will run for a whole day. That would be 1536 CPU hours. The largest jobs I see on our (relatively small) HPC are 128 CPUs for about four weeks straight. That's about 86,000 CPU hours. Most of our jobs are much smaller, sometimes fractions of a CPUh.
Jobs that need in the millions (Mio) CPU hours are truly massive, and would likely be running on the largest supercomputers around. You might find those jobs in weather forecasting, particle physics, government-sponsored encryption breaking, and similar applications.