In rule of thirds a picture frame is divided by four imaginary lines, two vertically and two horizontally, creating four intersecting power point.
Wrath by RawheaD Rex [Canon 5D II, 1/50, ƒ/22, ISO 200 @17 mm]
Visually, these four points would be the main focus points for human eye. Putting your main subject or interest on any of the points make an image visually more appealing and interesting.
Watch your horizon
This is part of rule of thirds. Placement your horizon is very important in composing landscapes. Its important the get the horizon straight. Well, you can correct it in post processing but it is a good practice to keep it straight. Avoid placing the horizon in the middle.