A
swing is a short to medium term price fluctuation that occurs
naturally in every financial market. Every trending or sideways
market is made up of multiple price swings. Price swings are caused
by fluctuations in demand and supply.
Swing
traders attempt to capture these price swings in order to make
a consistent profit in the market whereas investors and position
traders absorb these price fluctuations in search of the overall
trend. You can see an illustration of a swing here
and read an introductory tutorial here.