Open D is an alternate guitar tuning in which the strings are tuned to D2, A2, D3, F#3, A3, D4 (low to high). Starting from the lowest note, the interval structure corresponds to 1, 5, 1, 3, 5, 1. So in Open D tuning an D major chord sounds without the need of fingering.
The use of Open D (and its higher tuned twin Open E) is very common in blues songs featuring a slide guitar. Due to its low fundamental note D, tuning to Open D gives the guitar a full bodied sound which is also admired by many contemporary guitarists and songwriters.