The Draw Tool lets you draw on your photo, similar to basic paint programs.
Brush Settings

Brush Type
Use the Draw Brush to add brush strokes to the draw layer.
Use the Eraser to erase brush strokes from the draw layer.
Brush Color
Use the embedded color picker to select a color to paint with. Alternatively, you can use the Eyedropper button to grab a color from your screen, or use the Select Color... button to open the advanced color picker dialog.
Brush Size
Use the Size: slider to adjust the size of the brush.
Brush Hardness
Use the Hardness: slider to give your brush strokes hard or soft edges. Higher values will give sharper edges, and lower values will give softer edges.
Brush Strength
Use the Strength: slider to adjust the opacity of your brush strokes. Lower values are more transparent, and higher values are more opaque.