Pentre Ifan (PEN-dray EEV-an) is a Neolithic tomb a short walk above the Pembrokeshire hamlet of Felindre Farchog (vel-IN-dray VAR-[death rattle?]-og).
What you see is only the roof and two walls of the original tomb, which was completely surrounded and buried beneath a cairn of smaller boulders. It probably dates to ~3500 BC, and while it’s hard to say anything concrete about people who lived so long ago (excavations at this site seem not to have revealed much in the way of artifacts), the theory seems to be that this tomb would have contained the remains of high-ranking members of society. First the bodies would be left out to naturally mummify or rot down to bones in the sun and wind, and then the bones would be interred here, accumulating over multiple generations until the tomb was finally sealed up.
In the picture, you can just see how the roof stone rests on one stone at the end closer to us and two stones at the end farther away, without quite touching that fourth stone between them. That was the door; it would have had to be moved every time a new body was interred, then moved back into place (remember, the sides weren’t open at this time).
I can’t find a diagram. It makes more sense when you see it.