Overlinking in a webpage or other hyperlinked text is having too many hyperlinks (links). It is characterized by large proportions of the words in each sentence rendered as links, including links that have little information content, such as linking to specific years or dates; unnecessary linking of common words, for which the reader can be expected to understand the word's full meaning in context; linking to unrelated, obvious topics (e.g. linking to water in an article about sailing). One or more duplicate hyperlinks will almost certainly appear needlessly on the viewer's screen. The purpose of links is to direct the reader to a new spot at the point(s) where the reader is most likely to take a temporary detour due to needing more information. Providing more hyperlink samples for the same word in a short space (as in the example of this paragraph) doesn't help much.
Related concepts
The opposites of overlinking are "null linking" and "underlinking", which are phenomena in which hyperlinks are reduced to such a degree as to remove all pointers to a likely-needed context of an unusual term, in the text-area where the term occurs.