Literally "Okay, thanks, bye". Commonly used when short words and phrases are needed, as in when chatting and texting. Can also be used to quickly end a conversation, usually in a rude fashion.
Jane: "Hai John! Sup?"
John: "Nm, gtg, kthxbai."
Jane: "Hey John, why won't you talk to me?"
John: "Because. kthxbai."