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.