Select Screen at upper right, which makes the object a lamp and gives a black screen that can be dragged around.
Select Many rays at upper left to see how the lens bends some of the rays of light from the lamp.
With the lamp positioned far to the left of the lens, you should see that the rays that go through the lens converge to a point. If the screen is placed where the beams converge, the image on the screen will be in focus (it will be a small dot of light since that is what the object looks like in this case). The screen is then at the focal plane. As the lamp is moved closer to the lens, the distance between the focal plane and the lens _____.