Here is an excellent video explanation of how to use the ADX indicator.
The average directional movement index (A.D.X.) was developed in 1978 by J. Welles Wilder as an indicator of trend strength in a series of prices of a financial instrument. A.D.X. has become a widely used indicator for technical analysts, and is provided as a standard in collections of indicators offered by various trading platforms. https://en.wikipedia.org/wiki/Average_directional_movement_index
That was a nice overview of the ADX.