A tribe was a type of social group, usually smaller than a nation. They were common in civilisations that were not technologically advanced.
The Doctor encountered many human tribes throughout time and space, from the Tribe of Gum on prehistoric Earth (TV: An Unearthly Child) to the Sevateem and the Tesh in the far future. (TV: The Face of Evil) The Tribe of the Free lived on Ravolox in 2000000. (TV: The Mysterious Planet)
In South America in 1924, George Cranleigh encountered the Kojabe tribe and another tribe led by Latoni. (TV: Black Orchid) The Second Doctor, John and Gillian encountered a South American tribe and helped them fight off an enemy tribe. (COMIC: The TARDIS Worshippers)
Other species which formed tribes included Lakhotha, (PROSE: Scribbles in Chalk) Neanderthals, (TV: Ghost Light) Silurians, (TV: The Hungry Earth / Cold Blood) Swampies, (TV: The Power of Kroll) and the Sycorax. (TV: The Christmas Invasion) The First Doctor, John and Gillian encountered a hostile tribe on an Earth-like planet. (COMIC: The Didus Expedition)