Photographers often use thirds to imagine a grid when they are taking a picture. They divide the image into thirds horizontally and vertically. I have drawn these lines on the photo below and put red dots where the lines intersect.

Sometimes, a great place to put something important in your photo is right at one of the red dots. Do you see how I put my subject's face at the upper right hand red dot? This seemed to work better than positioning her face right in the middle of the picture. Also, the 2/3 water and 1/3 trees+sky in the background felt like a good choice.

Can you see how I used the rule of thirds in the photo below?

Remember there are no rules in art! Now that you know the rule of thirds, you should try using it sometimes and breaking it other times.