It is a score!
Quite simple. You can moderate other users' comments based on whether they contributed to, or are relevant to the article, or are just trolling and fishing for flames. This is then reflected in a (+/- value) score next to that user's name.

Note: Bitching about people moderating other peoples comments up / down in the comments system or complaining about the moderating system, especially if you try to pad your comment out with something that is on topic to the published article, will get your comment moderated down and / or moved to the off topic forums.
This is to keep the discussion relevant and on topic. You are still welcome to talk about whatever you want, but please do it in the relevant place.

For more information about comment moderation have a look at this page, and feel free to criticize it in our forums.