Kekw is deeper internet slang than most people are willing to discover. Kek is like a giggle or snort, not a full laugh but a chuckle. The w modifier means this is a win for them, so kekw is a chuckle about a good situation. If you want more info you can ask someone on 4chan, they are super helpful and nice to new members of the community.
Ik it’s not, but the only place you see it regularly is 4chan.