Thomas wants to host a Minecraft server for his friends. Is port forwarding secure? Port forwarding is where you tell the router to send traffic coming in from a specific port to a certain machine. This limits a little bit of the potential damage from opening up a server to the outside world, but it will ultimately depend on that Minecraft server to be secure. It's important that Thomas keeps his Minecraft server secure and up to date. If someone can figure out how to get around his network via the server, he could infect his network.
Leo recommends running the Windows firewall on all of his other machines inside the network, since that will isolate them from the Minecraft server. The biggest job here is keeping the Minecraft server secure though.