These are some examples of the sentences that describe the use of JK:
I just won the lottery! JK! I wish!
You are my best friend ... Jk! (But really, you are!)
I told my mother about going to Mars ... JK!
These examples show how JK is in subtitles - it simply means that you joked. For more details like jk meaning in chat, you can follow this link here you get more information.