It is your HN karma. You earn one karma point for every upvote you receive on a comment or submission. You lose one karma point for every downvote you receive o

It is your HN karma. You earn one karma point for every upvote you receive on a comment or submission. You lose one karma point for every downvote you receive on a comment. Everybody starts with one karma point. Some capabilities (e.g. the ability to downvote other people's comments) require you to have amassed a certain amount of karma. (I can't remember how much off-hand.)