Process Sent and Received Messages
Those numbers refer to Mach messages used to request and receive services from the system kernel. You'll have to grok Apple's kernel documentation to understand which kernel interfaces generate these messages and why. Unless you do low level system programming there is no reason for you to care about the message count numbers, which is why they aren't displayed by default.
These are messages sent between different running processes on your computer. It is normal for processes to send and receive messages.
It is not an indication of network traffic.