A tribe is a group of humans who live and work together, often possessing their own unique culture and beliefs. They usually are a community with common ancestors. Human societies in Horizon Zero Dawn are organized in the form of tribes.
Following the events of the Faro Plague, the human population was decimated. This resulted in the loss of their technological advancement and knowledge, as well as the regression of their civilization. In order to adapt to the natural world once again, human societies reshaped themselves into small groups of people, hoping to survive in a world ruled by the now-dominant machines.
These new tribes adopted a way of life similar to their ancient ancestors, hunting and gathering food and materials. Eventually, tribes grew larger, some even turning their villages into towns and small cities, and developed an economy based on resources obtained from machines.