Microsoft Knowledge Base Article 317530 contains the following summary:

Microsoft Windows Server 2003 provides Internet security in the form of a firewall, known as the Internet Connection Firewall (ICF). This feature is designed for home and small business use and provides protection for computers directly connected to the Internet. This feature is available for local area network (LAN) or dial-up connections. It also prevents scanning of ports and resources (file and printer shares) from external sources. This article discusses how to turn on (enable) the ICF feature to provide Internet security for your computer.