This policy setting allows you to specify if the Diffie-Hellman Group 5 protocol can be used by the Internet Key Exchange ...

"This policy setting allows you to specify if the Diffie-Hellman Group 5 protocol can be used by the Internet Key Exchange (IKE) protocol during Mobile VPN key exchange negotiations.  If you enable this setting, the Mobile VPN  can use Diffie-Hellman Group 5 key exchange.  If you disable this setting, the Mobile VPN  cannot use Diffie-Hellman Group 5 key exchange.  If you do not define this policy setting, the default behavior is to allow all supported groups.  Note: If all Diffie-Hellman groups are explicitly not allowed, the Mobile VPN fails."