Once in a long while someone will find a vulnerability in either Zope or Plone and they will release a hotfix for it. A hotfix is like any other product so just download it to some location on your physical server and unzip/untar it in your /Products/ directory.

Restart Zope through the command line or ZMI and you are good to go.

Of course, always make sure you have a backup before installing anything in your production environment and if need be you can un-install by just removing the product from the directory.