Meaning: the top part usually contains a description of a common (often even lucky) situation. In reading the bottom part, it is revealed that, in fact, something unexpected and usually disastrous happened. Other examples include:
Origin: The real name of the boy in the picture is Kyle (his last name was not publicly revealed). The photo was taken as his seventh grade school photo. It first appeared as a meme when someone created and posted the first image below to Reddit in January 2012 (DUI means “driving under influence”):
It wasn’t received well, but the second picture, posted later on the same day, got a very positive reception, and the meme became widespread since then.