What is "open tuning" on a guitar?
Standard guitar tuning - E A D G B E' - is very versatile, but sounds discordant when strummed with no strings being fretted.
However, if you retune to E, B, E, G#, B, E' and strum the open strings you will hear a chord of E - this is, for obvious reasons, known as "Open E" Tuning.
So, you can see that the term "open tuning" is fairly self-explanatory. It simply means that the open strings, un-fretted, play a harmonious chord. Now, it's not much good being able to play a single chords, but it's easy to see that if you now play a barre (with you first finger across all strings) on the 3rd fret, you'll get a G chord. Move the barre up to the 5th fret and you're playing an A chord. So open tunings make it easy to play the basic chords up and down the neck.
But the real power of open chords is in playing unusual chord inversions that would require near-impossible finger stretches if played on standard tuning. That's why open tunings are popular with composers and song writers. Just try out some open tunings and experiment - break the rules and enjoy it!
Open E Tuning - E, B, E G# B E'
Open A Tuning - E A E A C#' E'
Open G Tuning - D G D G B D'