It's "clean" install that doesn't erase the drive, essentially. The System folder, as well as the Library folder, are moved to a folder called Previous Systems, where they are unable to load at boot. Application Enhancer (along with the other few Preference Panes you have installed) will be removed and you'll have to reinstall them, but it means that neither they nor any of the other things like that you may have installed will be able to mess up your system. Your Users folder is untouched, except for the new things added in the upgrading process, so when you reboot, you won't have to go move your files back from a backup or anything like that, but you'll get the same fresh install that you would if you did an Erase and Install (which is what people are referring to when they say Clean Install, which hasn't been an option for many years).
jW