A more thorough explanation:

An Indian reservation is a specific area of land that the United States government has set aside for use by a Native American tribe. The tribe typically settles on this land and establishes a tribal government to govern the people who live there.

For example, the Navajo Nation Reservation is a large area of land in the southwestern United States that is designated for use by the Navajo tribe. The reservation covers parts of Arizona, New Mexico, and Utah, and is home to over 300,000 people.

Another example is the Standing Rock Indian Reservation, which is located in North and South Dakota and is home to the Standing Rock Sioux Tribe. This reservation gained national attention in 2016 when protests erupted over the construction of the Dakota Access Pipeline, which threatened the tribe's water supply.