LOL stands for "Laugh Out Loud".
It's a slang word used pretty much everywhere on the internet, so I'm kinda surprised you haven't heard of it before! Where have you been?
Anyway it's pretty much what most people use when they want to tell someone they're laughing at something they said over text or online.
Sometimes people just type "hahahahahahah" too... But that can be more time consuming I guess.
There are other ways of laughing online. Rofl, LMFAO.
By the way it's even used so much a lot of dictionaries have the word LOL in it!! LOL :)