Original: Signatures must include at least one internal link to your user page, user talk page, or contributions page; this allows other editors easy access to your talk page and contributions log.
Simplified: It is common practice to include a link to your user page or user talk page (often both); the default signature links to the user page. At least one of those two pages must be linked from your signature, to allow other editors to find your talk page and changes log easily.