An "off the shelf" product today means a commodity. I don't mean pork bellies or frozen orange juice. I mean things that you don't have to custom order. Ready-made things that are available in stores.
"Out of the box" means "as received from the manufacturer."
For example, you've bought a software package, say FLOTHERM 6.1 from MENTOR. You open the box, get the disk OUT OF THE BOX, and install it on your computer.
What has installed is out-of-the-box. All the default settings that you now have to change to keep your quirks happy are out-of-the-box default settings.
Out-of-the-box means what comes out of the box.