Winternitz One-Time Signature
$w$ is a parameter that can be freely chosen, to maximize performance.   Each element of the signature encodes $w$ bits of the message to be signed, so the larger $w$ is, the fewer elements you need to include in the signature.

If you make $w$ large, then signatures can be shorter; however, the tradeoff is that key generation, signing, and verification run slower.  If you make $w$ small, then signatures are longer; however, key generation, signing, and verification run faster.  So this is a tradeoff between the size of the signatures vs the running time of the scheme.  You can choose $w$ to provide the best tradeoff for your application.  The scheme will be secure no matter what value of $w$ you choose.