Artificial suns were artificially maintained stars. They gave off artificial sunlight.
The Martian Terraforming Project had intended to heat Mars with several artificial suns around its orbit once Mars was terraformed. This fell through when the Great Recession forced the project to be abandoned. (AUDIO: Deimos)
In the 23rd century, the Eighth Doctor caused an explosive feedback in the still intact atmospheric re-ioniser of the abandoned Martian project, causing both — according to one account — the terraforming of Mars and the ignition of Deimos Moonbase's atmosphere once the re-ioniser exploded. This chain of events turned Mars' moon, Deimos, into an artificial sun that could heat Mars' atmosphere. (AUDIO: The Resurrection of Mars)