The Bering Strait Theory is as follows. It is thought the first people in the Americas came from the Artic region of Siberia across a land bridge that is now under the Bering Sea. This occurred during the Ice Age, between 12,000 and 60,000 years ago. As water became locked up in the polar ice caps, sea levels dropped as much as 300 feet. The Bering Sea between Siberia and Alaska is no more than 180 feet deep and would have been dry land at those times. The bridge would have ranged up to a 1,000 miles wide, so the people crossing it wouldn't think of it as a bridge at all. The people that had been currently living in the Old World were nomadic big game hunters that followed the huge grazing animals of the period. The first people in North America followed the great herds of animals they hunted across the newly formed bridge to what is now Alaska.
After reaching Alaska they gradually spread out south from there and eventually reached the southern tip of South America, Tierra del Fuego by some 11,000 years ago.
The scientific community agrees (for the most part) that people never originated in North America, but emigrated from the Old world to the new many thousands of years ago. These theories go against First Nations culture and beliefs that they always lived in North America.