A packet basically helps your computer and the server communicate with each other. When packet editing, your editing the information that the server sends to you and a whole range of server sided hacks can be done via PE. Thing is, nexon likes to encrypt their packets so if you are trying to packet edit, you would first need to decrypt the packets you are recieving. I think i stumbled across a tutorial a while back, let me see if i can find it.
P.S: I haven't tried this on Combat Arms, so i might be wrong about encrypting/decrypting. But also keep in mind if you do any damage to their servers or cause the game to crash you can be found easily and charged. Or so I have heard...