Ascending and descending triangles are directional continuation (and sometimes reversal) patterns. Each pairs a flat horizontal level with a sloping trend line, and the slope tells you which way the pressure is building.
Ascending triangle
A flat resistance level on top with higher lows pressing into it from below. Each dip is bought sooner, compressing price against the ceiling. It usually resolves with an upward break — buyers absorbing the resistance until it gives. The flat top is the trigger level.
Descending triangle
The mirror: a flat support level with lower highs pressing down on it. Sellers cap each bounce sooner, and price usually breaks down through the floor. The flat bottom is the level to watch.
A caveat
The 'usual' break direction is a tendency, not a law — context still rules. In a strong uptrend a descending triangle can still break up (continuation beats the local shape). Trade the confirmed break on volume, and let the flat level serve as a clean invalidation if price reclaims it.