The Brush tool creates contour lines as you draw on vector layers, adding each brush line on top of the previous ones. On bitmap layers, the Brush tool creates 2D graphics, laying down individual pixels.
When you select the Brush tool, the Tool Properties view displays the different Brush modes that control how the brush line is drawn: Draw Behind and Auto Flatten. Click on the Brush Mode buttons to enable or disable the different states.
Draw Behind Mode
When enabled, your drawing strokes will be added underneath the existing artwork instead of over it.
NOTE Your stroke will appear over your artwork as you draw it, until you release the mouse cursor or tablet pen. If you want a live preview of your stroke, activate the Realistic Preview option by selecting View > Realistic Preview from the top menu.
Auto Flatten Mode
By default each brush stroke you draw is stored as a separate drawing object that can be manipulated individually. With the Auto Flatten option enabled, all overlapping brush strokes will be merged together into a single drawing object. This will make your artwork lighter in both file size and processing time, but may limit your ability to tweak and modify your artwork.