What is a site-to-site VPN tunnel?

A site-to-site VPN tunnel is where two firewalls are used to create a VPN tunnel. At Things Mobile we use IPSEC VPN tunnels. We configure a firewall on our side and then send the configuration parameters to the customer. So, a customer has to enter the configuration parameters on their firewall/router.
This allows any machine on a company's LAN to connect to its devices with our IoT and M2M SIM cards and vice versa. It is an always active VPN connection. To create a site-to-site tunnel, customers need to have a hardware firewall/router that can configure an IPSEC VPN tunnel.