A superseded patch is a patch that doesn't have to be installed because a later patch is available. A typical example is a service pack, which bundles many other patches that have been released before the service pack. If you install the service pack, you don't have to install all the earlier patches.
Patch Management only reports patches superseded by a service pack. Superseded patches have a string appended to the title of the patch that indicates that it is superseded by Service Pack X. This string is displayed as dark red text with a yellow background to make it stand out.
Example: Superseded By: KB936929 Windows XP Service Pack 3 (KB936929)
The installation process installs superseded updates only if the service pack that supersedes these updates is not selected for installation. If the superseding service pack is selected for installation, the superseded updates are not downloaded or installed. A procedure log entry is added to indicate the update was skipped because it was superseded.