[1] Interrupts are signals sent by hardware devices to the CPU alerting it that an event occurred to which it should react.
Most operating systems execute a HLT instruction when there is no immediate work to be done, putting the processor into an idle state.
On most architectures, executing such an instruction allows the processor to significantly reduce its power usage and heat output, which is why it is commonly used instead of busy waiting for sleeping and idling.
Since issuing the HLT instruction requires ring 0 access, it can only be run by privileged system software such as the kernel.
If every process is sleeping or waiting, it will normally execute a HLT instruction to cut power usage until the next hardware interrupt.