True range is the largest of: high minus low, high minus prior close, or prior close minus low. ATR averages that over 14 periods by default. It is expressed in price, not percent.
ATR is the workhorse for position-sizing and stop-loss placement because it adapts to the instrument. A 2-ATR stop on a quiet stock and on a volatile one represent similar amounts of normal noise.
Example: a stock at $80 has a daily ATR of $2.40. A stop 1.5 ATR away is $3.60 below entry. Risking $300 means a position of 83 shares.
Related: volatility, position-sizing, stop-loss, trailing-stop