The percentage retracements identify possible support or resistance areas, 23.6%, 38.2%, 50%, 61.8%, 100%. Applying these percentages to the difference between the high and low price for the period selected creates a set of price objectives. Depending on the direction of the market, up or down, prices will often retrace a significant portion of the previous trend before resuming the move in the original direction. These countertrend moves tend to fall into certain parameters, which are often the Fibonacci Retracement levels.
From mod @zAngus