This is a very simple example of an image that is being broken into nine equal squares to be tiled. The corners stay in the corners, and the middle parts get repeated.
The different “tiles” that the image is broken into do not need to be the same size, but the ones in the middle of each side do need to repeat, and preferably they will not be particularly long. Keeping them short increases the likelihood of the tiling being seamless all the way across.
For more general info and some examples, see this page:
For more about how the tiling works, including a helpful diagram: