The Average Directional Movement Index is an indicator used to measure the strength of an existing trend. You can see this indicator as a simple line moving up and down to show the strength of the trend. However, this indicator does not give you the direction of the trend. The default setting for this indicator is 14. It also consists of two lines, +DI line, and the -DI line.
When the ADX line is below 25, it is considered as no trend. Between 25 to 50 is considered to be in a trending situation in the market. Above 50, is considered as a strong trend.
For example, look at this chart. The market breaks out of this bullish flag and then slows down. The ADX also shows lack of strength in the trend by divergence. So, it is time to take profits. You can also consider taking a short position. The best way to use this indicator is to look for the confirmation of an existing trend and use other technical indicators for buy and sell signals.