The rule of thirds is a guideline that will make your landscape and outdoor scenes more dynamic and give them more energy.
Imagine your scene as being divided into nine equal parts. The horizon runs along the bottom third of the photo. Your subject should be placed on the on the intersection of two lines, which is usually called a crash point.
In the photo below, Prairie Memories, the abandoned farmhouse is the main subject in the lower right. But you'll note that the cloud above is also placed near the intersection of two lines in the upper left corner. The open spaces of blue sky really make the farmhouse pop out of the photo.
The rule of thirds is an easy and simple rule of thumb to remember and use. Experiment with it and see if you can bring more life to your photos!