Each pullback stops higher while sellers keep defending the same price. The reading is that supply at the level is finite and buyers are willing to pay more each time, so the level eventually gives way.
It completes on a close above the horizontal boundary. The measured target is the triangle's height added to the breakout, and the stop usually sits below the last higher low.
Note that the flat top is also a row of equal-highs, which means a cluster of stop orders. Breakouts from ascending triangles frequently spike, fill those orders, and then fail. A rule requiring the break to hold for a defined time or to be retested handles this better than entering the instant the level trades.
Related: descending-triangle, symmetrical-triangle, equal-highs, false-breakout, measured-move