This was used to avoid massive HTML cache invalidation when a page is edited, and a large number of other pages depended on it, e.g. a highly used template.
It was used to place a threshold value on when to skip cache invalidation if there were more than
$wgMaxBacklinksInvalidate number of dependent pages, and a warning was output.
If this variable was unset, (or set to the default
false) then the threshold was hard-coded to a default, 200.
This variable has been removed and replaced with $wgJobBackoffThrottling with MediaWiki 1.23.0.