A user's "karma" reflects how much a user has contributed to the Reddit community. When posts or comments get upvoted, that user gains some karma. You can see how much karma a user has on their profile page.
The best way to gain karma is to submit posts that other people find valuable and interesting. Being on-topic, relevant, funny, interesting, or engaging are great ways to earn comment karma. Being insulting, rude, or abusive is not. Snark can go either way. You can learn more about karma here: How does karma work?