The kk expression, like many other Internet expressions, is part of online conversation culture. "kk" stands for "Ok" or "message acknowledged". It is the same as nodding in person or saying "gotcha". "Np" means "no problem!"
The other bit of history behind "kk" is the 1990's expressions "k, kewl".
Translated, this expression meant "ok, cool", but was stylistically spelled otherwise. "k, kewl" undoubtedly also influenced the use of kk in today's online chatting.
Texting, Twitter, chat and IM abbreviations and acronyms represent people's shorthand communications via mobile devices and on the Internet, especially on social media platforms. Find texting, Twitter, chat and IM abbreviations and acronyms at TechTarget.