The act of just writing k in a text/im. People generally don't like responding to such a small answer thus being k'd generally ends the conversation. If you are the one being k'd, you should assume the person that k'd you is either very mad at you, or very pissy.
1: Hey, are you ok? You seem upset.
2: I don't wanna talk about it.
1: Well I'm here if you do
1: *thinks to them self* Wow, I can't believe he just k'd me! I'm not answering that!