The lasso brush combines the lasso selection and paint bucket tools. You can find it in your left toolbar. If you are in Super Simple Mode, then your Lasso Fill tool is grouped with the Paint Bucket. Press and hold on the bucket icon until the menu appears with the lasso option.
As you draw a shape with this, the area will be filled with color. A straight line is drawn to close the shape between where you started and the current position of the cursor. Lifting your pen will complete the shape.
In the settings panel you can set the fill to be solid or patterned, adjust the opacity, and activate astabilizer.
Setting the lasso file mode to erase will erase an area as you draw it out.
Enabling auto-erase activates the lasso fill in erase mode when you press and hold the shortcut for your eraser