A 4chan meme/trolling technique in which a user posts a picture of a well-known celebrity (such as Michael Jackson, David Bowie, or Will Smith) and places birthdate and the current date, along with the line Goodnight, Sweet Prince as text in the picture. The intended purpose is to trick you into actually googling said celebrity to see if they really did pass away.
Most people don't fall for it, although I admit to curiously searching for recent news of a few celebrities to check if they did, in fact, die. (Fool me once, shame on you. Fool me twice, shame on me.)
The origin of this phrase comes from William Shakespeare's Hamlet:
"Now cracks a noble heart. Goodnight, sweet prince. And flights of angels sing thee to thy rest."