It means that they are mostly independent from other Imperial organizations.
While the crusading chapters fight with the whole righteous wrath of the Emperor followed by the support (including logistics) from Adeptus Mechanicus, Imperial Guards, Sisters of Battle etc, Space Sharks (Carcharodons) work alone with minimal contact with other imperial groups, to such degrees that even Inquisition barely knows about their existence.
When they need equipment, they trade with Adeptus Mechanicus (aka. Grey Tithe) or take anything that is not nailed down from the battlefield, when they need new recruits or serfs (Red Tithe), they either kidnap all the imperial rejects they can get (prisoners, rebels, orphans etc) or again, they trade - and they don't see anything wrong with working with renegade chapters.
The Sharks are a fleet-based chapter. At some point in their history, Rangu (their word for the Emperor) or the Forgotten One (either the Primarch of the origin legion or the Founding Chapter Master) banished/self-exiled them from their home world and set them to hunt amongst the stars and slaughter the enemies of the Imperium with no quarter. To this day, no one knows why they were banished. To this end, they were granted the rite of Tithe; the Grey, all the physical material they needed for their duty, and the Red, a claim to prisoners, rebels, rejects, and other detritus for ship crew and potential recruits. They are explicitly forbidden from taking serving members of the Adeptus Terra (read - anyone from any other branch of the Imperium) in their tithing. The Grey Tithe is usually a semi-regular swap meet with allied members of the Mechanicum, bartering a few pieces of archeotech for freshly made materiel.