An uptrend line connects at least two rising lows and is extended forward; a downtrend line connects falling highs. Two points define the line, but most traders want a third touch before treating it as meaningful, because any two points can be joined.
Traders use trendlines for three things: describing the angle of a trend, placing entries at a touch, and treating a decisive break as a warning that conditions changed. A steeper line usually means a move that cannot be sustained; a shallow line often marks a slower move that lasts longer.
Be honest about the weakness. Trendlines are drawn by hand, so two people rarely draw the same one, and the temptation to redraw a line after it breaks is enormous. Decide in advance whether you are using bodies or wicks, whether the chart is on log-scale or linear-scale, and never move a line just because price went the wrong way.
Related: trend-channel, trendline-break, support, resistance, log-scale