A trendline is a straight line drawn on a price chart to show the direction of a trend. You draw it by connecting two or more swing highs or lows.
An upward trendline connects rising lows and can act as support while an uptrend holds. A downward trendline connects falling highs and can act as resistance while a downtrend holds. The more times price touches the line without breaking it, the more traders watch that level.
A trendline is diagonal, which sets it apart from a horizontal support or resistance line drawn at a fixed price. A trendline tracks a moving level that rises or falls with the trend, while horizontal support and resistance sit at one price. A break of the trendline can flag a weakening trend, but traders confirm it with price action, volume, or indicators before acting.
EUR/USD prints higher lows at 1.0800, 1.0850, and 1.0900.
You draw an upward trendline connecting these three lows.
If EUR/USD later pulls back and holds above the line, you can treat it as support. If price breaks below the line, you may read it as a bearish signal.