Imperialism is, according to google, a policy of extending a country's power and influence through diplomacy or military force. Many countries would fight over land in Africa. Meaning they would walk on to unknown land and claim them. If the Natives would resist they would force them with weapons and military force.
In Avatar you will see many examples of a civilization with imperialism. For instance, the Na'vi was introduced a new language and the humans also built them a school to learn. You also will see the humans constructing roads. In African, they were introduced to new roads and schools as well.