RotMG Packets

I would like to know about RotMG packets as I hear this is important to learn about. Can someone explain to me in-depth about packets? I would like to know the significance of learning them, what they are and how they work. And finally what I can do with them, modifying, deleting and maybe introducing packets somewhere in the code to make new hacks or modifications.

Packets are the collections of information that the server and client use to communicate. Man-in-the-middle hacks like K-relay modify packets coming from the server to the client and going to the server from the client. For example a spam filter in K-Relay might check incoming Text packets from the server and simply not send them on to the client if they contained strings that were classified as spam.