The rule of thirds is one of the most basic composition guidelines used in photography it is a great way to create a strong visual impact easily, it draws upon the natural tendency for the human eye to be drawn into particular parts of the image, this helps you to make sure your followers/viewers will focus on the area you want them to in your image.
It is basically a grid dividing your image into 3 equal parts, horizontally and vertically, you will have created a grid of 9 boxes with the lines intersecting at four equal points, according to the rule of thirds it is where the lines intersect that you should place your the parts of your image that you want the viewer to look at, these are the ideal places for points of interest.
Here is an image that I took applying the rule of thirds, I wanted the viewer to be drawn to the centre of the Rose, as we read from left to right I tend to place my object on the left hand side of the composition, only because it is a more natural way for the human eye to move, for cultures in the world who read from the right, then it would be more natural to place your your subject on the right hand side of the image.
As you can see, I have used the top left hand cross point to place the centre of my rose. Using this method it allows you to to also place your focus point either up on the top cross section point of the lower, depending on your subject.
Most Cameras and modern mobile phone camera settings allow you add a grid onto the screen so that you can utilise this rule of thirds whilst composing the image. Alternatively you can take your photo then apply the rule of thirds whilst cropping your image. I personally prefer to do it in camera rather than at the editing stage. After a little while you will probably by able to judge quite easily with just your own eye and no need to use the grid at all.
Remember however it is only a guideline, as they say, rules are made to be broken, but if you are just starting out it is a super easy way to create a lovely visual impact!