What is the "Don't Say It" meme?
The meme isn't brand new but has spread like wild over the weekend. It plays on the tension between things you know you shouldn't say but want to say anyway. It's right there in the name. Your mind says "don't say it," but you just can't help it. It represents a sort of metaphorical disconnect between your brain and your mouth.
It might be easiest to think of Michael Scott on The Office. Anytime he was told not to say "that's what she said," this meme would be a good representation of what's happening in his head.
At one point on the show, Michael was forbidden from telling sexual jokes. Jim Halpert tempted him by saying, "You always left me satisfied and smiling." Inside Michael's head, he was probably repeating "don't say it." Nonetheless, he's eventually powerless and has to say, "That's what she said." In this meme, that exchange looks something like this.