A retracement is a temporary reversal in a dominant upward or downward trend.
From the screenshot I attached, the price is in an upward trend but the yellow circle shows the moment a retracement happens after which the upward trend continues strong.
Some traders erroneously exit their trades when they see a retracement because they think the trend is ending, only for the trend to continue.
Some traders also trade the retracement, so they look out for it and trade in the direction of the retracement.
The best way I identify retracements is watching out for when price approaches major resistance/support levels or using the Fibonacci retracement tool.