In the US, this is a general term for descendants of Native American peoples. Their degree of sovereignty unfortunately varies; some tribes are "federally recognized" and enjoy state-like recognition of their autonomy by the federal (central) government, while others have to bargain with the states surrounding their territories. More info here:
https://en.wikipedia.org/wiki/Tribe_(Native_American)And a FAQ: https://www.bia.gov/frequently-asked-questions
The land they directly control can be anywhere from tiny to gigantic, depending on the particulars: https://upload.wikimedia.org/wikipedia/commons/e/ef/Indian_L...
Many people also live in surrounding areas: https://en.wikipedia.org/wiki/List_of_Indian_reservations_in...
Much of the time the tribes have their own small governments, similar to municipal services, but also often with their own equivalents to "courts", "police", etc., who may utilize different corrective measures than the US ones. Often they will have an agreement with surrounding law enforcement (or the federal government) to partner up on certain categories of infractions, such as traffic violations or murders.
Some tribes have some money thanks to casinos and other business activities, but most are unfortunately quite poor, and many of their communities must work very hard to survive.
(This all is just my layman's understanding. I am not Native American, but I've lived near their communities. Please correct me if I'm wrong!)
---------
Edit: The 2023 movie Killers of the Flower Moon is really worth watching: https://en.wikipedia.org/wiki/Killers_of_the_Flower_Moon_(fi...