One of the first, if not the first, true sword & sorcery setting where the prominent character of Conan lived.
A low-fantasy setting, inhabited mainly by humans though a few beings, like demons, also show up.
It is a very cruel, brutal, raw setting as Robert E. Howard wrote it, with Conan being an archetypal anti-hero.
The Hyborian Age is a fictional period of Earth's history within the artificial mythology created by Robert E. Howard, serving as the setting for the sword and sorcery tales of Conan the Barbarian.
The word "Hyborian" is derived from the legendary northern land of the ancient Greeks, Hyperborea, and is rendered as such in the earliest draft of Howard's essay "The Hyborian Age." Howard described the Hyborian Age taking place sometime after the sinking of Atlantis and before the beginning of recorded ancient history. Most later editors and adaptors such as L. Sprague de Camp and Roy Thomas placed the Hyborian Age around 10,000 BC. More recently, Dale Rippke proposed that the Hyborian Age should be placed further in the past, around 32,500 BC, prior to the beginning of the Last Glacial Maximum. Rippke's date, however, has since been disputed by Jeffrey Shanks, who argues for the more traditional placement at the end of the Last Glacial Maximum.
Howard had an intense love for history and historical dramas; however, at the same time, he recognized the difficulties and the time-consuming research needed in maintaining historical accuracy. By conceiving a timeless setting – a vanished age – and by carefully choosing names that resembled our history, Howard avoided the problem of historical anachronisms and the need for lengthy exposition.
Source: Wikipedia, "Hyborian Age", available under the CC-BY-SA License.