They're the caretakers of Sigil. They tear down the rotten buildings and build up new, sound ones. They trim back the razorvine, and dump corpses in the Ditch (Sigil's only body of water, if it can be called that).
They sweep the streets and nail covers on the portals to the plane of Ooze. They pave over sinkholes and feed the prisoners of the Mazes. They push garbage into the Plane of Ooze. They bring buckets of sunlight into the city during the day, and buckets of darkness and moonlight at night. They transmit messages from the Lady of Pain. After the Faction War, they run the city courts through former members of the fraternity of Order.
They can help visitors by giving them directions and so on, if they choose to and if the visitor can understand them.
They don't have any particular abilities. They would be fairly easy to kill, except that someone who murdered one of her servants would be punished by Her Severity, the Lady of Pain.
Despite the efforts of the dabus, Sigil is mostly not a clean, safe, or orderly place. The Collectors are still needed to gather corpses, and razorvine and unexpected portals to the Plane of Ooze can still snatch the unwary. Perhaps they deliberately concentrate on the Lady's Ward and ignore the Hive Ward, or perhaps the parts of the Hive Ward that are fixed up drift into the cleaner parts of Sigil. Regardless, there really aren't enough dabus to do a throrough job. The dabus are what keep the city from falling apart, however.
When their shifts are done, the dabus float into tunnels beneath the streets, where their secret nests and rumored hidden city lies. Some have speculated that if you go deep enough, the solidity of Sigil ends and it turns out that the entire metropolis is built on holographic glyphs generated by the dabus.
There is a race native to a demiplane in the Ethereal called the phirblas. They resemble dabus except they seem to be physically younger, and they speak through floating, holographic words instead of rebuses. Their demiplane, Inphirblau, is a nigh-infinite city. It is thought that the phirblas are descended from the dabus, or vice versa.
No one has ever seen young dabus or phirblas, and if they have different genders no one can tell them apart.