A pre-shared secret is a secret key that is shared between the two VPN peers before establishing a secure connection. It is used to authenticate the VPN peers and encrypt the VPN traffic. A pre-shared secret is required for a site-to-site VPN tunnel that does not use certificates, because certificates are another way of authenticating the VPN peers using public key cryptography. Without certificates, the VPN peers need to have a common secret key that only they know. Reference: Check Point R81 VPN Administration Guide, page 13