Some skins are comprised of matching html and ascx files - the html file has simple tokens that then get parsed when the skin is uploaded/installed to the portal, to produce the matching ascx file, which is what DNN uses to actually display content through the skin.

So, they are saying that if you only make changes to a skin's html file and upload it, you won't see any of your changes reflected because the html "tokens" need to be parsed into the matching ascx file. The only way they get parsed is by installing/reinstalling the skin.

Keep in mind that some skins don't even have html files as they aren't necessarily needed, but only used if present to produce the ascx files.

Also keep in mind that if you DO only make changes to an ascx file, all you have to do is ftp upload it to the site to see changes because it is in it's "final form" for immediate use on the site.