Information
A firewall is a piece of software that blocks unwanted incoming connections to a system. Apple has posted general documentation about the application firewall.
http://support.apple.com/en-us/HT201642
Rationale:
A firewall minimizes the threat of unauthorized users from gaining access to your system while connected to a network or the Internet.
Impact:
The firewall may block legitimate traffic. Applications that are unsigned will require special handling.
Solution
Perform the following to turn the firewall on:
Graphical Method:
Open System Preferences
Select Security & Privacy
Select Firewall
Select Turn On Firewall
Terminal Method:
Run the following command to enable the firewall:
$ sudo defaults write /Library/Preferences/com.apple.alf globalstate -int <value>
For the <value>, use either 1, specific services, or 2, essential services only.