These pages are indexed by search engines because they include the magic word
__INDEX__ in their wiki source.
Using this keyword in your user page (or your talk page) authorize crawlers visiting this wiki and finding a link to user user page (or to your talk page) to index it and return it as part of their search results.
Normally most pages in a wiki are indexable, with the exception of user pages and all talk pages (in their respective namespaces) that are excluded by default (for privacy reasons).
See mw:Help:Magic words#Behavior switches for details.
Content pages in category "Indexed pages"
The following 42 pages are in this category, out of 42 total.