Here's how it's used:
* To express humor: "I just saw the funniest cat video ever! Lewl."
* To acknowledge a joke: "You're so funny! Lewl."
* To show that you find something amusing: "My friend just sent me the most ridiculous meme. Lewl."
Essentially, "lewl" is a way to convey a chuckle or a lighthearted laugh in a quick and informal way.