One of the uses of IPSec is to creat a private tunnel between two places. This could be two FireBricks, or between a FireBrick and some otehr device such as a router, VPN box, linux box, etc.

The tunnel allows traffic to IP addresses at the far end to be routed over the Internet in secret, encrypted at the sending end and decrypted at the receiving end.

To set up a tunnel, configure IPSec in the tunnel set up, setting the same algorithm and secrets at each end, and specifying the IP of the far end. You then set up routes for blocks of IP addresses to be routed down the tunnel - each end setting the routes to the far end.