A part of a computer system or network which is designed to block unauthorized access while permitting outward communication..

The description provided refers to a "Firewall". A firewall is a network security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. It establishes a barrier between a trusted internal network and untrusted external network, such as the Internet.

Here are the steps a firewall takes to control traffic:

1. Packet Filtering: The firewall checks the packets (small amount of data) against the filters. If the packet matches the filter, it is allowed to pass, otherwise it is blocked.

2. Stateful Inspection: The firewall checks the state and context of the packet. It not only looks at the packet but also at the connection. If the connection is secure, the packet is allowed to pass.

3. Proxy Service: The firewall acts as a proxy and receives the packets on behalf of the recipient. It then inspects the packet and if it is safe, forwards it to the recipient.

4. Circuit-Level Gateway: The firewall checks the TCP handshaking. If the TCP handshaking is done properly, the firewall allows the packets to pass.

5. Application-Level Gateway: The firewall checks the application data of the packet. If the application data is safe, the packet is allowed to pass.