Virtual Home Button

I am unsure of the uses of the virtual home button. Can somebody enlighten me on how to use this virtual home buttton effectively?

I've also observed that when I touch the button, it transforms into a bigger, black box with other buttons, a few of which I have no idea about their purposes and how to use them properly, such as "3D Touch", "Device" and "Double-tap". If you can help me out with this, it would be much appreciated.