As reported in the [manual][1], "chaining" is a parameter that controls the way in which lines are rendered.

 If enabled, Blender will draw each line as if it was coonected with the previous; on the other way, if disables, it will render each line **separately**.

You can see the effect by increasing line thicknes and zoom in:

[![enter image description here][2]][2]

While the same file with chaining enabled led to:

[![enter image description here][3]][3]


  [1]: https://www.blender.org/manual/render/freestyle/parameter_editor/line_style/strokes.html#chaining
  [2]: https://i.sstatic.net/MCAB9.jpg
  [3]: https://i.sstatic.net/WkCmK.jpg