In a normal Doom game, the players' input is sampled 35 times per second (see tic). These inputs are placed in structures called ticcmds. The ticcmd structures are sent as packets to the other computers in the game.
If a packet is lost, the receiving machine transmits a resend request to the sending machine. Under Doom, the game cannot advance until all players have received the ticcmd for the next tic to be run. Packet loss therefore causes a short stall in the game. This affects the perceived "smoothness" of the game.
Over a large-scale network such as the Internet, packet loss is a common occurrence and therefore a serious issue to multiplayer games. If the extratic option is enabled, the Doom engine includes the previously transmitted ticcmd when sending a new ticcmd. The result is that if a single packet is lost (the most likely occurrence), it will not have such a large effect on the game, as the next packet will contain the same ticcmd, removing the need to retransmit.
The drawback to the extratic option is that each ticcmd structure is effectively transmitted twice, increasing the amount of bandwidth needed. In the worst-case scenario, the bandwidth needed doubles (although this is often not the case as the header data of the encapsulating protocol is not affected).