Alternate guitar tunings are an amazing way to change things up on your guitar. You'd be amazed at how much simply dropping the low E by a tone can change the character of your guitar... and if you keep on tinkering further it keeps on changing.
Standard guitar tuning is simple E A D G B E, and when we talk about alternate tunings, we're simply changing one of those string tunings.
The ability to use alternate guitar tunings is one of the things that makes the guitar one of the most (if not THE most) versatile instruments out there. Some artists have spent most of their careers pursuing odd tunings, and become quite accomplished at playing in them. Keep in mind that ever time you change the tuning on a string, any chord or scale patterns that you used previously change according to how you've modified that string tuning.
Another thing to keep in mind when you change the tuning on your guitar is that by altering even a single string, it can change the tension on the whole neck, and you may need to re-tune the whole guitar after dropping or raising one of the strings.