The caching tag should be used when asking about how SE caches web pages and data. Also for when encountering any perceived errors/bugs that you think have to do with caching.
Caching is displaying old information (not-always-instantious-updated-info) on the website pages, in order to increase page speed load times and provide a better user-experience. Generally, most things are not cached for that long on SE, except for maybe the help topics where the information on the page doesn't change very often.
