Göbekli Tepe, or "potbelly hill" in english, dates back to somewhere between 11,000 to 12,000 years ago. The site was rediscovered by an archaeological team in 1963, but it was thought to be from to be from much later, and not much was made of the discovery.
It wasn't until 1994 when archaeologists began digging around it to find out more, when they discovered the columns of T shaped pillars.
It is the widespread belief that this site was built by hunters and gatherers who had not yet domesticated animals, and had no knowledge of agricultural techniques. They likely lived in makeshift villages nearby for at least part of the year.
The site predates Stonehenge by about 6,000 years, and was built before people had "developed metal tools or even pottery."
The site has remained a mystery- and it has posed questions about the development of civilization. It was previously believed that humans started settling down after the agricultural revolution.
The site has been regarded by many as the world's oldest temple. "Scholars have long believed that only after people learned to farm and live in settled communities did they have the time, organization and resources to construct temples and support complicated social structures."
This site suggests that "the extensive, coordinated effort to build the monoliths literally laid the groundwork for the development of complex societies." Source
This discovery doesn't overturn what historians and archaeologists believe, but it does make it appear that it's possible that "sociocultural changes come first, agricultural changes come later."
It is also interesting to note that there were many carvings of animals and insects such as snakes and scorpions. "Scholars cannot interpret the pictograms, and do not know for certain what meaning the animal reliefs had for visitors to the site; the variety of fauna depicted, from lions and boars to birds and insects, makes any single explanation problematic."
New discoveries have suggested the site may have been used for human sacrifice, and the site could have been the center of a "skull cult" where skulls were venerated after death. Anthropologist Julia Gresky explains "archaeological remains from other sites in the region indicate people would commonly bury their dead, then exhume them, remove the skulls, and display them creatively."