A graviton field is any area in which many gravitons may be found. Graviton fields are used in the production of artificial gravity. They are also found naturally-occurring in space.
Generally speaking, natural graviton fields appear to be more powerful than man-made fields (for example, see the one encountered by the USS Enterprise-D in 2367).
In 2369, the graviton field of Deep Space 9's tractor beam was able to stabilize Tosk's ship's structural integrity. (DS9: "Captive Pursuit")
Several weeks later, a lifeform from the Gamma Quadrant created a graviton field that drew DS9 towards the wormhole. (DS9: "Q-Less")
The USS Voyager encountered a natural graviton field in 2373. (VOY: "Future's End").
Lenara Kahn and the Trill Science Ministry came up with a way to seal the Bajoran wormhole using a modulated graviton field. (DS9: "In Purgatory's Shadow")