The neckline turns a picture into a rule. Until price closes through it, the pattern is only potential; once it does, the pattern is considered complete and the measured-move target is taken from it.
Necklines are not always horizontal. A sloping neckline is common and changes the geometry, which is one reason different traders reading the same chart get different targets. Decide whether you measure from the break point or from the flattest version of the line, and be consistent.
Broken necklines frequently get retested, and that retest is the entry many traders prefer because the invalidation-level is right there. The trade-off is that in the strongest examples price never comes back and you miss the move entirely.
Related: head-and-shoulders, inverse-head-and-shoulders, double-top, measured-move, retest