Trendlines define the dominant direction of a market. In an uptrend, you draw a line under successive higher lows. In a downtrend, you draw it above successive lower highs. Price pulling back to the trendline is often a high-probability entry zone in the direction of the trend.
Trendlines are subjective — two traders drawing them on the same chart will often disagree on the exact anchor points. Most prop traders treat trendlines as zones (10–20 pip wide bands) rather than precise lines, and require at least three touches before considering the trendline valid.