Americans are usually protected by their constitution from things like random searches, detainment, etc. without a warrant or probable cause. These rights don't exist at the border. As anyone who's visited the United States from away knows, they, their car, their suitcases, etc. can be searched by border patrol officers.
National security. Fair enough.
Now, according to today's article, the American government says that the border exists from the actual border to anywhere within 100 miles of that border. According to the U.S. Census Bureau, two thirds of Americans live within 100 miles from a border. So, two thirds of American citizens are subject to random searches, as though they were entering the country.
If you want to know if that includes, you, the ACLU provides a map of the "constitution-free zones".