When using the rule of thirds the main focus of the photo will be on one side this gives us a more interesting photo but an unbalanced one. It is unbalanced because the focus on one side makes the other one feel empty. You can balance and image by adding a subject of lesser importance then the main focus.
There are 2 types of balance in photography. One is formal balance also called symmetrical balance. It is when one or more identical or similar subject are repeated symmetrical on each side. The other one is informal balance which can also be called as asymmetrical balance. This balances the photograph by adding a dissimilar subject to it. A well taken photograph that uses informal balance is very hard to achieve but you will be able to get once you have used it many times. You are able to achieve a good balance if you are able to add the right amount of colour, shapes, lighting and shadows.