For those who have not seen the movie, and don't want to have it spoiled for them, I advise you stop reading now.
So many of you may be wondering what the post-credit scene in X-men DOFP aka. [X-Men: Days Of Future Past](movie:203942) was all about, and some of you may have left before it was shown. So allow for me to explain it piece by piece.
So the scene starts with a mass crowd gathered in a desert bowing towards a man and chanting at him. What they are chanting is the name "En Sabah Nur". Who is En Sabah Nur you may be wondering? Well, En Sabah Nur is actually the birth name of the worlds first mutant. A mutant who later on became to be known as the supervillian "Apocalypse", pictured above.
We see Nur levitating various parts of a pyramid, and is shown to be piecing them together to form a pyramid. In the comics, Apocalypse uses an ancient magic to power the pyramids to try and turn all of mankind into mutants. From this we can assume that this is exactly what he is doing in the scene.
In the last shot of the scene, we can see that Nur has a blue tint to his skin, an obvious nod to his form in the comics.
We can also see 4 men in the distance sitting on horses. This is a nod to the 'Four Horsemen of Apocalypse'.
In the comics, Apocalypse takes control of 4 mutants to be his 4 horsemen of Death, Famine, Pestilence and War. The most famous of the mutants he has controlled over his many incarnations has been the X-man Angel, who he brainwashed to become Archangel, the Horseman of Death.
With 'X-men-Apocalypse' confirmed for 2016, this scene was a great teaser for what we can expect from [X-Men: Apocalypse](movie:1194267).