VPNs can be configured in two modes:
Always On (Default)
This mode will try to keep the VPN up at all times. If that is not the case, connections through the VPN will be rejected.
On-Demand
In this mode, the VPN connection will be established when needed. If it is not needed any more, it will be shut down after 15 minutes of inactivity. This helps to save resources on not very frequently used VPNs.