This is one of the primary tools used for drawing. You can configure the brush size and fill style using the controls in the toolbar.
To draw using this tool, simple left click and drag the mouse.
When using the Solid Brush fill style, the primary color is used. If you are using a different file style, both the primary and secondary color will be used for the different parts of the pattern. If you use the right mouse button to draw instead of the left mouse button, the roles of the primary and secondary colors will be reversed.
The following example shows the brush being used to draw a brush stroke in two solid colors and two patterns.
Paintbrush Tool: Two solid colors, two patterns