An up fractal is formed when a high is both preceded and followed by lower highs. A down fractal is formed when a low is both preceded and followed by higher lows.
Bill Williams uses a period of 2 (2 after and 2 before) to define a fractal. This indicator has 2 as default but lets you decide the range you want.
The indicator lets you also select alerts when price crosses an up of down fractal, which would usually mean to entry a position. This indicator works best when used with an alligator, as long signals are valid above the Balance Line and short signals are valid when triggered below the Balance Line.