What it means
Ascending (flat highs, rising lows) leans up; descending leans down; symmetrical can go either way.
In practice
A triangle shows a market coiling: each swing is smaller than the last as buyers and sellers close in on agreement. Traders wait for a close outside one of the lines and enter in that direction, with the stop inside the pattern.
The widest part of the triangle projected from the breakout gives a rough target. A breakout in the direction of the earlier trend is the more common outcome.
What to watch for
Breakouts that happen very late, near the tip of the triangle, tend to be weak. False breaks of one side followed by a real move out of the other are common.