With Mambo, a patch refers to a minor upgrade that is usually released to correct bugs or fix security issues that have been identified in a major release or branch of the Mambo code.  Occasionally, a patch may include some minor improvements on features.

Patches are numbered incrementally. For example, Mambo 4.6.1 is a patch for the Mambo 4.6 version.  Further improvements to Mambo 4.6 were released in the patch called Mambo 4.6.2.

To patch or not to patch?
Any patch that includes security fixes should be applied as soon as possible.  Announcements about patch releases always state whether the patch includes security fixes. 

While it is always a good idea to run the latest stable release, it is not always necessary to upgrade your sites.  Patches usually include some bug fixes.  If, however, you are not experiencing any problems with the way you use your site , and the patch does not include any security updates, you can stay with your current Mambo version.  Keep in mind, however, that the longer you remain on your version, the older it gets and the upgrade path may become more complicated as a result.