A horizontal line is a charting tool that marks one fixed price level across time. It flags a price where the market has reacted before, so you can watch it on the next approach.
A horizontal line has zero slope, because it sits at a single price. Traders draw it to mark support, a price area where buying may slow a fall, and resistance, a price area where selling may slow a rise.
A horizontal line differs from a trendline, which slopes to track a rising or falling market. The horizontal line stays flat at one price, so it suits ranges, breakouts, and retests; pair it with candlestick patterns, volume, or moving averages for confirmation.
You analyse EUR/USD and see price react several times near 1.1000.
You draw a horizontal line at 1.1000 to mark resistance.
If EUR/USD rises to 1.1000 and rejects it, you may treat it as a sell area. If price breaks above 1.1000 and holds, you may read it as a bullish breakout.