The statement refers to the concept of stateful packet inspection. In Stateful Packet Inspection (SPI), the firewall examines and tracks the state and context of packets that pass through it to determine whether to allow or block the packets. SPI maintains a record of the state of active connections, including the IP addresses, ports, and sequence numbers of packets exchanged between the hosts. If a packet from an external computer arrives that does not belong to an active connection, the firewall will block it. This protects against threats such as port scanning and denial-of-service attacks that attempt to exploit open ports or flood the network with traffic.