The shape says buyers tried twice at the same price and failed twice. The trough between the peaks is the neckline; the pattern is not considered valid until price closes beneath it. The conventional target is the height of the pattern projected down from the break, a measured-move.
Volume traditionally falls on the second peak relative to the first, which is read as weaker participation. That is a nice story and sometimes true, but it is not a reliable filter on its own.
The complication is equal-highs: the same picture is also a pool of stop orders above the peaks, and a spike through them followed by a collapse is a very common outcome. That version traps breakout buyers rather than short sellers, so both sides of the pattern can hurt.
Related: double-bottom, triple-top, neckline, equal-highs, measured-move