If you use a garden hose for instance and lay it in a pool of stagnant water, the water can be syphoned back into your system.
A cross connection devise simply blocks the water from moving back up the hose. You can buy the part at any hardware store and it attached to your outside faucet. The same can happen with your home dishwasher I believe.
I'm not a plumber but the guy who installed mine explained it like that.