The Cultivator was the ancient force of balance and order in the Starbound universe. It existed for aeons before the rise of the Ruin, and under its influence, harmony reigned. With the rise of the Ruin, however, the Cultivator was forced into a fight for the survival of the universe. Although it was able to seal the Ruin away from the rest of the universe, it exhausted all of its energies doing so. This caused it to be destroyed in the process.
Before the Cultivator dissipated altogether, it gifted six chosen races with powerful artifacts. These were the Hylotl, Human, Avian, Floran, Apex and Glitch races (every playable race exept Novakid). These artifacts, when all placed in the gate to the Ruin, will open the way, allowing the Ruin to be defeated for good.
The Cultivator is depicted as humanoid, and is implied to be astronomically large, being depicted holding or looming over planets and galaxies. Whether it actually was such a form, or if it had any physical form at all, however, is not established. There is a theory that the Ancients worshiped it as a god, or possibly even could commune with it on a higher level than other beings, but that they only personified it in their religion and it was a mere force. This has yet to be proven, however. Both known depictions of the Cultivator are located at the Ark. It's also possible that the Cultivator was simply revered as some great leader to the Ancients rather than a deity-like figure, but they evidently hold great respect and have some connection to the Cultivator.