From Halopedia, the Halo wiki
A daemon is a computer program that performs its function "in the background" during the normal operation of a computer system, for example providing security services, performing maintenance operations, communication with peripherals or other subsystems, or more.
During the Battle of Installation 00, the daemons in the Ark's computer systems were alerted to the presence of the formerly imprisoned AI Mendicant Bias, which had reconstituted itself using the lost fragment aboard the Covenant's Forerunner Dreadnought. The daemons attempted to confine Mendicant and prevent it from communicating with John-117 via the terminals, although they were not wholly successful.
Historically, daemons, not to be confused with demons, are good or malevolent supernatural beings between mortals and gods, such as inferior divinities and ghosts of dead heroes.