What it means
A third touch confirms it. A trendline break is a warning, not a reversal; structure confirms.
In practice
A trendline gives a trend a slope. Traders use it to see whether price is keeping its pace and to find pullback entries when price returns to the line together with other evidence.
A channel adds a parallel line through the opposite extremes, showing where a trend tends to be stretched. A steeper line is easier to break and tends not to last.
What to watch for
Trendlines are subjective: two traders will draw them differently, and a line can always be redrawn to fit. Stops also collect just beyond a clean, well-touched line, which makes it a target.