A trendline is a line drawn along swing lows in an uptrend or along swing highs in a downtrend. It shows a possible area where the price may pull back in the future.
Some traders also opt to buy during an uptrend when the price pulls back and then bounces higher off of a rising trendline, a strategy of buying the dip. Similarly, some traders elect to short during a downtrend when the price rises to and then falls away from a declining trendline.
Trend traders will also watch for chart patterns, such as flags or triangles, which indicate the potential continuation of a trend. For example, if the price is rising aggressively and then forms a flag or triangle, a trend trader will watch for the price to break out of the pattern to signal a continuation of the uptrend.