ATR is a measure of volatility. It shows you how much a particular stock moves (on average) in a day over a certain period of time (adjusted for gaps). A trader can use ATR to place a stop-loss on a relevant technical place. Placing your stop-loss at two or tree ATR reduces the chance of being unnecessarily stopped out due to normal “fluctuations”.
In Chartmill you can add an ATR based stop to your charts by adding a “chandelier exit” to your chart.