Just like the classic game on your phone (eat things without hitting yourself or the walls), but in 3D.
You would think that this game would have an issue when the volume of the snake exceeds that of the box, but by pressing 'v' you will see that things become very difficult long before then; I usually feel the squeeze when the volume of the snake gets to around 0.03 (or 3%) of that of the box.
If the applet draws a box and red tube, but doesn't do anything when you press spacebar, you may need to click on it a few times to give it focus.
Here is the java source.