The rule of thirds is a basic photography technique. Divide a picture in nine equal parts (two equally spaced horizontal and two vertical lines). The important compositional elements of the image should be placed along the intersections of horizontal and vertical lines. This will create interest, tension and balances your picture.
You can also use the technique in landscape photography. The horizon should either touch the upper or lower horizontal line. So it covers one third (and two thirds for landscape) or two thirds (and one third for landscape) of your picture.
Once you failed to nail the rule of thirds in camera you can at least try to find it in post production. The cropping tool can be of help to get a straight horizon, to get it on a horizontal line or sometimes even to put your main subject on an intersection.