I believe the belief you're looking for is that the Native American's believed that the world belonged to everyone and couldn't be bought or sold. The Europeans didn't agree with this, but they did take advantage of it. A lot of the time when Native American's signed contracts selling their land, they didn't know they were actually selling it to the Europeans, they thought they were merely sharing it.
Hope this helps! Please let me know if I didn't answer your question :)