The NG Firewall Packet Test is a packet capture utility accessible via Config > Network > Troubleshooting > Packet Test. This uses the tcpdump utility for network troubleshooting in the GUI. There are two modes: Basic and Advanced. Basic mode is a very basic test to check traffic from a specific host/network, via a specific port (not required), and on a specific interface (for viewing internal or external traffic from the specified host/network) . This also includes a timeout amount for how long the test will run.
The advanced mode allows for configurable arguments for a more granular view of traffic, including multiple hosts, multiple ports/protocols, and configuring output from the utility. This can be rather complex, and we suggest researching the tcpdump man pages, or other online resources for viewing how to use tcpdump.