Manual:$wgAdvancedSearchHighlighting

Details
Set to true to have more useful text snippets in search results: $wgAdvancedSearchHighlighting basically does not really change the way a search hit is highlighted. In so far the option is poorly named. What it in fact does change is which part(s) of a result page get(s) displayed: Without $wgAdvancedSearchHighlighting set, irrelevant parts might show up while with $wgAdvancedSearchHighlighting set to true, the places of the exact hits are more likely to be displayed.

Off by default due to execution overhead.