With Computer Games that use Serials/CD-Keys as protection, there are two types of keys.
Legal keys are Online Keys, and Offline Keys are those found via less aboveground means.
An offline key should not be seen by the software company's Keyserver, because it will know the key in use is a pirated key. This is a Bad Thing. Once it identifies that the key in use is bad, it can lock the software, expulse you from their servers or practically anything the manufacturer wants to... depending on the application of course. Just knowing that you have an illegal key is a bad thing too.
Thus, the "offline" part of all this.
Because very few people stay offline while gaming, and only play games single player, Offline keys are generally used with direct dial, or direct TCP/IP sessions, while the Keyserver's IP has been blocked via firewall, so no communication with the keyserver occurs, even though all players are playing online.
Or, you could simply play on a LAN with no router.