Originally defined to mean "laugh out loud" but is now so overused that no one is actually laughing when they type it. It is just something that people say when they have nothing else left to say. Often the words "hahaha" are substituted for it to give variety. People also type emoticons
such as: ":]" to also fill in the space when they get tired of "laughing out loud" or not "laughing out loud". It is the most used word when texting, and one of the easiest to type in order to send a message to someone so that you don't look like your blowing them off.