Simply turn on the Firewall, and select the exceptions for Web Access, iTunes, and Bonjour. If you need more, you can always add more, or just turn off the firewall, since if you are behind a router at home, it is difficult for anyone to see your system on the internet.
Stealth Mode - If your comptuer recieved unsolicited traffic, it will ignore it. Meaning, that if someone is scanning a network for computers, they will not be able to see yours since your computer will not respond the his pings.
TEG