What does this song mean? Who are johnstones? Me no get it.

       D          
  The Laddies by the banks o' Nith
       Em                A
  Wad trust his Grace wi a', Jamie;
  But he'll sair them, as he sair'd the King 
        G            D
  Turn tail and rin awa', Jamie.
  
  D                A
  Up and waur them a', Jamie,
  Up and waur them a';
       G                  D
  The Johnstones hae the guidin o't,
       G               D
  Ye turncoat Whigs, awa'!
  
  The day he stude his country's friend,
  Or gied her faes a claw, Jamie,
  Or frae puir man a blessin wan,
  That day the Duke ne'er saw, Jamie.
  
  But wha is he, his country's boast
  Like him there is na twa, Jamie;
  There's no a callent tents the kye,
  But kens o' Westerha', Jamie.
  
  To end the wark, here's Whistlebirk,
  Lang may his whistle blaw, Jamie;
  And Maxwell true, o' sterling blue;
  And we'll be Johnstones a', Jamie.