If you have a scene that consists of a simple color and you want to know the average color of the photo, it is easy to see what the color is at any one point by using a low number of samples and then averaging the result.
However, if you have a more complex scene (with a character for example), and use only a few samples, sometimes half of them will be yellow and half of them blue. Sometimes all the samples will be one color or the other. Sometimes, by coincidence, all of the samples will be on the yellow character, despite only a part of the scene being the yellow character. A way to improve the average is by increasing the number of Camera (AA) samples.
Increasing the number of Camera (AA) samples resolves the noise and improves the quality of the image.
The diagram below shows a good method for identifying the type of noise in your scene and which sampling parameter to use to improve it.