This is the cache of the HTML generated by Wiki pages.
If you write in a Wiki page
this WikiSyntax has to be parsed so that Tiki sends to browsers something like
normal, < i>italic< /i>
This parsing is done by tikilib's monster function parse_data.
When you cache Wiki pages, the parsing is done once and the HTML output saved in database for further accesses to that page. The next parsing will occur after the "global cache" setting time. Parsing again after some time avoids that the output becomes outdated (the parsing output depends on the rest of the Tiki).
You could probably set a cache of one day for mature sites. Hardly hit pages (c.e. HomePage, etc.) can be set more carefully with individual cache.