This menu item will mark the currently selected result. This menu item is a toggle, so if the current result is already marked, then it will unmark it.
Marking a result means that it's relevant, and all other results and words will have their relevance recalculated based on the new marked result. Their relevance is a measure of how similar the other results and words are to the marked result. Results similar to the marked result, and words associated with the marked result, will have higher relevance than other results and words.