They are the martian equivalent of the geosynchronous orbits (GSO) on the Earth.
A satellite in areosynchronous orbit does not necessarily maintain a fixed position in the sky as seen by an observer on the surface of Mars; however, such a satellite will return to the same apparent position every Martian day.
If a satellite in areosynchronous orbit were to be used as a communication relay link, it "would experience communications ranges of 17,000 to 20,000 kilometres (11,000 to 12,000 mi)" to various points on the visible Martian surface.
[1] An areosynchronous orbit that is equatorial (in the same plane as the equator of Mars), circular, and prograde (rotating about Mars's axis in the same direction as the planet's surface) is known as an areostationary orbit (AEO).
Although no satellites currently occupy areosynchronous or areostationary orbits, some scientists foresee a future telecommunications network for the exploration of Mars.