Abbreviation for "I laughed too hard at this". Example, sending a meme or joke to a friend.
NOTE: "ILTH" not "ILTHAT" for confusion of the phrase "I love that".

Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.