The pattern shows an trend losing strength: a high (left shoulder), a higher high (head), then a lower high (right shoulder). A close below the neckline completes it. An inverse head and shoulders is the bullish version at a low.
The conventional target is the height from head to neckline projected below the break. The pattern fails often enough that the neckline break, not the shape, should be the trigger.
Example: shoulders at $95 and $96, head at $105, neckline at $90. A close at $89 completes the pattern with a measured target of $90 - $15 = $75.
Related: trend, flag-pattern, resistance, breakout