The opening range is simply the high and low of a given period after the market opens. This period is generally the first 30 minutes or the first hour of trading.
During this period, you want to identify the high and low of the day. In addition, you will also want to account for the pre-market highs and lows, as these levels will often act like a magnet on price action after the bell rings.
Since the opening bell is associated with big trading volumes and volatility, this time of the session provides many trading opportunities. In this manner, traders use the opening range to set entry points on the chart and to forecast the price action for the day.