Each number has an explanatory tooltip.
![478 [vba] x 123: Asked 29 non-wiki questions with a total score of 272. Gave 93 non-wiki answers with a total score of 478](https://i.sstatic.net/KFgBw.png)
So, that 42 is your net score for the c++ tag (which counts toward your tag badge), and the x13 is the number of posts you have in that tag (questions & answers combined).
Tag score is the number of upvotes minus the number of downvotes under a given tag. Because only non-wiki answer upvotes count toward a tag badge, that's what the numbers are showing.
More information on tag badges hereMore information on tag badges here