CPU throttling occurs when the CPU is under enough load and unable to keep maximum turbo for the number of utilized cores (usually all cores). Throttling happens in one of four ways:
Thermal throttling occurs when the CPU heats up to a threshold, either one set by the OEM(Original Equipment Manager), or set across all CPUs of that generation (Ex: all 8th gen CPUs throttle at 100°C), and begins reducing the clockspeed (also reducing power draw), reducing thermal output. Thermals outside of manufacturer specification reduces CPU lifespan and is NEVER a good thing.
Power Limit Throttling
Power limit throttling happens when a system is not configured to provide enough power to the CPU to allow it clock up under load. All 8th gen CPUs' default power limits (set by Intel) are too low to stop throttling from happening under normal circumstances, so manufacturers that set or allow higher power limits to be set are relatively commonplace.
VRM/Current Limit Throttle
VRM throttling is a blanket term for a wide range of problems, but most commonly the core components on a motherboard are insufficiently specced, insufficiently cooled, or a combination of both or the CPU cannot draw enough current(measured in amps) to fully boost to a maximum clockspeed. It is difficult to diagnose, and usually unfixable even if diagnosed without replacing the motherboard.
This is the only throttle that happens under ideal conditions. This mean the CPU downclocks when not stressed to reduce power draw and thermals. This is the only throttle reported to the operating system as throttling by the cpu.