A regional sports network is a television channel that provides sports programming for a specific geographic region or locality. This means that you will see coverage for a team(s) that is located in your region. For example, if you live in the tri-state area (New York, New Jersey, Connecticut) and are a baseball fan, you'll see coverage of New York Yankees and New York Mets.
Unfortunately, if you're a fan of a team outside of your area, you're not going to be able to see many of their games, if any. This makes platforms that give access to all regional sports networks desirable, as you do not have to be in a specific location to enjoy games.