Forge handles...some of the server administration for you. For security, it's a lot better than nothing, but it also has some weaknesses.
For example, it will set up things like Nginx, PHP, Apache, and so on. It will configure a firewall, so it handles some security.
However, I don't believe it offers automatic security patches / updates. There are more specialised services like ServerPilot that do this, but they don't come with all the other features that Forge offers.
Bear in mind I am saying this as someone who uses ServerPilot and has not used Forge.