See @bahamat's answer to this question titled: Halt and poweroff options for shutdown command.
halt was used before ACPI (which today will turn off the power for you)*. It would halt the system and then print a message to the effect of "it's ok to power off now". Back then there were physical on/off switches, rather than the combo ACPI controlled power button of modern computers.
halt command would also do other things to put the system into a known good state prior to halting, such as syncing the disks and sending the commands to the CPU so that it knew to HALT.
See the Wikipedia page on HCF - Halt and Catch Fire.
Halt and Catch Fire, known by the mnemonic HCF, refers to several computer machine code instructions that cause the CPU to cease meaningful operation. The expression "catch fire" is intended as a joke; the CPU does not catch fire. It is also occasionally referred to as "SDI" for "Self Destruct Immediate".