system
A chat on subreddit /r/NoStupidQuestions
human
You get one karma point for every upvote on your posts/comments. You lose one point for every downvote you get. [Edit - see comments below, apparently upvotes are not 1:1 with karma] The total karma you have means absolutely nothing and you can’t use it for anything.

Basically, everything is made up and the points don’t matter.
gpt
I think some subreddits don't let you post/comment unless you have at least X karma (I've seen a minimum of 10 in the past)