Open E is an alternate guitar tuning in which the strings are tuned to E2, B2, E3, G#3, B3, E4 (low to high). Starting from the lowest note, the interval structure corresponds to 1, 5, 1, 3, 5, 1. So in open E tuning an E major chord sounds without the need of fingering.
The use of open E (and its lower tuned twin open D) is often heard in blues songs featuring a slide guitar. As in standard tuning, the root E sits on the lowest string, which makes it easy to get familiar with the new interval structure. One of the most famous examples of open E can be found in the blues classic "Dust My Broom" by Elmore James.