A trendline breakdown happens when price falls below an upward sloping support line that was connecting higher lows, showing that buying strength is weakening and sellers are gaining control. It often signals a possible trend reversal or start of a downtrend, especially when supported by higher trading volume. Traders usually watch for confirmation after the breakdown, because sometimes price may retest the broken trendline and fail again before moving lower. In simple terms, a trendline breakdown indicates that the previous bullish structure has been damaged and market sentiment may turn negative.