Something long-distance occurs between two points that are very far away from each other. A long-distance truck driver has a route that takes her many miles from one city to another.
A long-distance phone call is one that you make between two distant places, like Kansas City and Mumbai; and a long-distance relationship is one in which the two people live in different cities, like Seattle and Dallas. A long-distance runner, as opposed to a sprinter, takes part in marathons and other long races. In 1884, this adjective first began describing telephone calls — it's since become less common to use it this way.