This video shows you how to tune your guitar to Open D tuning (D-A-D-F#-A-D). Open D is the same relative tuning as Open E, just a whole step lower. In general, I recommend using Open D rather than Open E as it usually results in better action and fewer broken strings. (If you need to play in the key of E, you can use a capo on the 2nd fret.) Open D tuning is popular among slide guitar and fingerstyle players – great for country / blues riffs, new age fingerstlyle, slack key, just about anything really. I hope you have fun exploring the possibilities of this tuning.
If you want to explore more of this tuning, you can check out my lesson on Barcelona, by George Ezra, which is a fingerstyle song in Open D.
You may also be interested in my lesson on how to tune to Open G.