This can mean two different kind of Dances:
The old meaning is for a dance where couples form into lines, Men on one side and Ladies on the other for dances such as the Virginia Reel. Thes dances are characterised by movements such as Casting out, Arches, Advance and Retire in Lines (e.g Bridge of Atlone) and Strip The Willow.
The newer meaning is a type of American dance for individuals where everyone dances in a line or series of lines, a formation or sequence of moves. The music is normally Country and Western or Bluegrass.