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Note: Only some namespaces are searched by default. Try prefixing your query with all: to search all content (including talk pages, templates, etc), or use the desired namespace as prefix.

That's a really boring way of saying if you were trying to fiddle with some sort of special content, you might need to fiddle different namespace settings. Generally, at this site, you won't want to leave the Main namespace since that's where all the content lives.