The Old Ones were the oldest and most powerful psychic race in the galaxy. They had become the dominant power in the galaxy up until the War in Heaven, when they fought against the Necrons and their C’Tan overlords, who were immune to their psychic powers and were actively consuming their souls. The Old Ones fought back by creating the Orks, who alongside the Eldar, drove back the Necrons.
The aftermath of the war caused the Realm of Souls to mutate into the Warp, causing daemons and enslavers to kill the surviving Old Ones. It is unknown if any Old Ones are still alive, but their artifacts still remain, such as the Webway.
In TTS, the Emperor describes the Old Ones as a race of toadmen who are constantly irritable and senile, and are shown using rocking chairs. Likewise, their rocking chairs are used to represent the technology they left behind.