What is 'TMI'? What Does This Expression Mean?

You've just been making your first forays into online gaming, and you are "listening" to other players chat to each other. You see these odd expressions like "rofl" and "kk".  Then something funny happens and you see "TMI!" appear in the chatbox. What exactly do these obscure expressions mean? What is "TMI?" Well, rest assured, these expressions are not swear words. Quite the opposite, actually. Here is what TMI means...