Tanith is a direct descendant of the conceptualizations of Venus in early science fiction, which in turn were generally based on ideas in fiction about African jungles etc. It is hot, humid and home to various large and ferocious animals which are happy to prey on humans. There is one large landmass on an otherwise oceanic world. It is covered in jungle with few roads and those hard to maintain. Gravity is somewhat greater than Earth's. Hurricane-like storms are frequent, but the local plant life is well adapted to them.
In the period of the Falkenberg's Legion stories (late 21st century), Tanith is a prison planet. Since almost anything will earn a person a prison sentence in this time, the incoming population features both common criminals and intellectuals. Convicts find themselves under a system of "inheritable debt-indenturement contracts" --legalese for slavery. The convicts are used as labor to produce borloi, a marijuana-like drug which keeps the population of Earth placid. The few convicts who eventually finish their sentence (or their parents') have nowhere to go, and are absorbed into the local economy. The alternative is to take their chances in the jungle, which some manage to succeed at, some existing in stockaded villages, others pursuing brigand lifestyles under the rubric of Free Staters. However it is also easy to fall victim to one of the many large carnivores, or succumb to malnutrition or fungal infection. Tanith's insects, on the whole, do not bother humans.
Tanith is the setting for crucial events in The Prince, where the government of the planet Sparta, in alliance with the CoDominium Navy, begins its consolidation of military power by allying with Falkenberg and his mercenaries.
Tanith is also mentioned in passing in The Mote in God's Eye. Admiral Plekhanov mentions to Rod Blaine that his father had almost gotten both of them killed on Tanith. As it happens, the Governor of the Tanith in The Prince is one Carleton Blaine, evidently an ancestor of the Blaines of Crucis.