What to see
Above there is a brick wall, no doubt. Anything special? Do you see the object which would not be expected in such a wall?
When I first saw this, it took me considerable time to find it. When this image went viral in 2016-05, many people complained they couldn't find it, so if you can't find it: That's normal.
What to do
First: if you have not found the object, enjoy this feeling – Because, once you've seen it, you can never unsee it.
If you now want to see the object, move the slider slowly to the right. Or, if you're in a hurry, press the space bar or click on the image. Behold!
Explanation: Our brain continually reconstructs an “inner world” from incomplete information, relying on memory (what is likely in a scene?). Here, the object in question is rather unusual in a brick wall. Furthermore the photo is taken from such an angle that the object aligns with the mortar gap. But once you have seen it, the slight deviations from a mortar gap proper become obvious. Furthermore, your brain has changed (“one shot learning”): This experience has changed the probabilities of what can appear in a brick wall. [Vision scientists call this the Bayesian approach to perception.]
There are may pictures of this sort, usually presented as puzzles. Famous ones are “Napoleon”, the “Cow”, or, on this site, the hidden Dalmation and the Hidden Bird.
I have not been able to locate the original source. The earliest source I found is 2012-03-25 on reddit (see image on the right), although I cannot access the original thread anymore. Perhaps someone knows more here?