The Brush and the eraser tools are used to create (using the brush) and remove (using the eraser) masks or to edit masks. Masks is what we we call shapes that are more complex than polygons and bounding boxes. A mask is set on a pixel-level, and is used for when you need pixel-perfect level annotations.
The brush tool can be selected by pressing the icon or by pressing "B".
The eraser tool can be selected by pressing the icon or by pressing "E".
When selected, you use the brush as you would use a brush in any graphical editing software (for example Microsoft Paint). Move your mouse to where you want start painting, click and hold down the "left mouse-button" and then drag the mouse pointer to paint.