The Airbrush tool simulates painting with an airbrush or a spray can.
To use this tool...
After selecting the colour you wish to use, click on the Airbrush button and point the cursor where you want to begin painting. Hold down the left mouse button to paint with the foreground colour; hold down the right mouse button to paint with the background colour. Paint with the brush by moving the mouse. To change the size, opacity, and flow of the airbrush, double click on the Airbrush button.