What is comment_karma?

While doing a var_dump on the $comment object, I see there is a comment_karma property. What is that?

You get the comment_karma property because there is such a column in the comments table. It is meaningless and AFAIK was never used for anything except for maybe some random spam prevention plugin. It is there just for backward compatibility and because no one thinks that removing it will improve anything in a measurable way.

You can use that field if needed, but it is probably a better idea to use your own meta field.