Okay kiddo, sometimes people in different parts of the country want to watch TV shows that aren't available where they live. So, the government lets some stations in some areas 'share' their shows with stations in other areas. This helps people who live far away from the original station still watch their favorite shows.
These 'shared' stations are called "significantly viewed" because lots of people like to watch them, even though they're not in the normal local area. So, if you're on vacation in another state and you want to catch up on your favorite show, you might be able to find a significantly viewed out-of-market television station that carries it.
But don't get too excited, because not all shows are available on significantly viewed stations. Only a few selected ones are allowed to be shared. And these special stations may have different commercials or news broadcasts, because they cater to people in different areas of the country.
So, to sum it up for you: people in different parts of the country can watch some TV shows from stations that are far away, because the government lets these "significantly viewed" stations share their programs.