Each Green circle represents positive one and each red circle negative one. Use the sliders to change the number of circles. You can take away negatives (red circles) using the Subtract slider.
This is just one way at looking at some examples of subtracting negatives. Don't "overthink" it! You can also use the number line to explain this, there is another applet to show this in the links for this book. See http://tube.geogebra.org/m/2068193