Open G is an alternate guitar tuning in which the strings are tuned to D2, G2, D3, B3, B3, D4 (low to high). Starting from the lowest note, the interval structure corresponds to 5, 1, 5, 1, 3, 5. So in open G tuning an G major chord sounds without the need of fingering. Note: The root G is on 5th string, not on the lowest 6th string as in most other tunings.
The use of Open G (and its higher tuned twin Open A) has a long tradition among blues musicians playing a slide guitar. Other guitarists like Keith Richards took even greater advantage of its simplicity when fingering chords. He took off the 6th string of his Telecaster and composed tracks like "Honky Tonk Women", "Brown Sugar" and "Start Me Up" with it.