Wireshark-users: [Wireshark-users] Connecting to router

From: Rohan Solanki <solanki.rohan@xxxxxxxxx>
Date: Sun, 26 Apr 2009 23:27:24 -0700
Hi all,
  While i was searching on the web of "how to connect wireshark to a router", I found the following link

http://www.plus.net/support/broadband/troubleshooting/wireshark.shtml

In this link, in the 3rd step, in the 3rd sub-step, it states that "Select the relevant network interface from the drop-down at the top of the Options window. If you are using a router this will be your Network (NIC) card."

Can anybody explain this statement...
How do i connect wireshark to a router, so that i can view the packets that are flowing through the router?

Thanks in advance
Rohan