This French phrase is painted in simple script on Rene Magritte's 1929 painting "La trahison des images" ("The Treachery of Images") right below the depiction of a plain brown smoking pipe. It translates as "This is not a pipe." Because, well... it's not, it's just a painting of a pipe.
A coffeeshop on the UIUC campus has a small painting on the wall behind the counter, identical to "The Treachery of Images" except with these words instead:
Ceci n'est pas une Magritte
I never could convince anyone else that this was hilarious.