A horizontal trendline support zone is a price area (not just a single line) where the market repeatedly finds buying interest at roughly the same level, causing price to pause or bounce upward; it’s drawn by marking multiple similar swing lows and shading a small band to account for market noise and wicks. Traders treat this zone as a high-probability demand region—looking for bullish candles, volume confirmation, or other signals near the zone for potential long entries—while placing stop-losses just below it, because a clean breakdown with strong momentum often signals trend weakness and can open the door for further downside.