Noise breaks up brush strokes and helps to make brushes look more realistic and less computer generated.
Global noise
Global Noise slider is controlled by adjusting the slider bar at the top of the Brushes Library, and this setting effects all brushes in the library.
The higher the value, the more noise is added to the brush stroke. The image below shows a brush stroke of 0% noise, 50% noise and 100% noise:
Individual noise settings
The Noise setting in the middle of the Brushes Library sets the noise level for the selected brush only, and overrides the global noise setting at the top of the library.
Note: All settings applied in the Brushes Library, whether they are global settings (at the top of the library) or individual settings, are automatically saved. You do not need to take any action to save them.