Williams %R is an oscillator.
It helps determine the direction of the price of an asset and find the reversal points of a trend.
Williams %R can move between 0 and -100. However, it periodically crosses -20 and -80.
Crossing -20 signifies that the price of an asset has reached too high a level.
Crossing -80 signifies that the price of an asset has reached too low a level.