The letter ㅋ sounds like 'K' so its just an expression as if they are laughing "ke ke ke" they also use 하 하 "ha ha". Its like when we say lol

Thank you Jack, that's helpful!^^

ᄏᄏᄏ = Sound of grinning, ᄃᄃᄃ = *shivers*, ᄐᄐᄐ = *runs*, ᄒᄒ = Sound of laughing, ᄇᄇ = bye bye, ᄒᄋ = hello, ᄀᄀ = Let's go!, ᄂᄂ = No, ᄋᄋ = Yes, ᄌᄌ = Good Game, ᄋᄃ? = Where?, ᄀᄉ = Thanks, ᄀᄃ = Wait, ᄆᄎ = Crazy, ᄀᄎ = It's okay, ᄁᄌ = Get lost, ᄌᄉ = Sorry, ᅲᅲ = Crying, ᅩ = F**k you!, ᅳᅳ = *Stares at you*, ᄉᄉ = *smiles*, ᄆᄂᄋᄅ = herp derp (meaningless sounds).. took it from http://www.memrise.com/course/62531/chatting-online-with-koreans/

Haha no problem, it gets quite annoying when you see it constantly and you don't know what it means! "ㅋ ㅋ ㅋ"

Thank you for asking this, I was wondering the same thing!