They are a set of indicators developed by Welles Wilder that are plotted with three lines ADX, DI+ and DI-.
The ADX is calculated from the Directional Movement Index.
The Directional Movement Index (DMI) is decomposed into DI+ (Positive Directional Index) and DI- (Negative Directional Index) and is a measure of the amount of movement upward or downward that have the prices.