Ages ago when texting was new, lol was born. Then there came the phenomenon of WoW. In world of warcraft, when the horde players would ambush and kill alliance players they'd often trash talk afterwards. However, you couldn't understand what they were saying, it was gobbledygook. The one thing that translated very easily though was "kek" often accompanied by a /laugh emote. Kek was "orcish" for lol. Thus Kek was born.

Tldr: Kek means lol.