A trendline breakout and retest happens when price decisively breaks above a downward trendline (or below an upward trendline), signaling a potential change in trend or continuation of strength, and then pulls back to test the same trendline from the other side. This retest is important because it shows whether the old resistance has turned into support (or old support into resistance), confirming market acceptance at higher levels. If price holds above the trendline after the retest and buyers step in with good volume or bullish candles, it indicates stronger conviction and often provides a low-risk entry opportunity with a clear invalidation point.