We have just removed the X-Score from the user interface. It was not properly explained and does not bring enough value to merit keeping it.
X-Score is an internal algorithm which ranks results when you select the “Most Relevant” sort order (which is the default). The X-Score number represents how important each individual result might be, and moves it up or down in the result list.
In the old version it showed the X-Score on the right side, next to the date:
The current version does not show the X-Score anymore.
If you want to know how our search interface looked like last year, check out this blog post.