In FortiManager the admin can monitor and configure multiple FortiGate units in one console. In Device Manager, you can view, configure, and make changes into the FortiGate devices without having to log in and out of multiple FortiGate units.
The below video showcases the step to integrate multiple FortiGate devices into Fortimanager.
By going to the Device Manager, FortiManager enables you to complete the configuration. All changes to the FortiGate configuration are stored locally on the FortiManager unit until you integrate with the FortiGate unit.