Average True Range measures an instrument's typical volatility over a set number of periods by averaging the true range (the greatest of: current high minus low, high minus previous close, or low minus previous close) of each period. Unlike RSI or MACD, ATR says nothing about direction — only about how much an instrument tends to move.
ATR is widely used to set stop-loss and take-profit distances that scale with current volatility, rather than using a fixed pip distance that might be far too tight (or too wide) depending on market conditions.
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Guide
Who Decides Where a Range Begins and Ends
Ranging, quiet and sideways are statements about a window somebody chose. What fixes a range's boundaries, why a session high is a fact about a convention, and why the parameter is part of the finding.