Navigation Links allow you to quickly navigate between related pieces of code. They highlight particular code fragments, and allow you to easily tab through them in both directions. Once you've tabbed to a link, the target code fragment is automatically selected, so you can immediately start typing over it.
For instance, the Add Parameter refactoring automatically creates Navigation Links in all updated method calls. This allows you to quickly traverse through all the places that require your attention, and pass proper values to the newly added parameter.