From Halopedia, the Halo wiki
Lag, from "lag behind", is a slang term for slow Internet speeds or Internet latency; Lag is sometimes due to a server problem, but more frequently due to the connection between client and server.
A lagger (occasionally designated as a High Ping Bastard or HPB) is someone who uses a relatively slow 56 kbit/s modem to connect to the Internet and through Xbox LIVE, then joins a multiplayer online game (usually a first-person shooter) and who therefore has a high ping time. In synchronous games, where all clients must know the exact game state, a single "lagger" will slow down the game for everyone, since the server has to wait for the client to get every communication packet. However, many online games use an asynchronous communication model, allowing the clients to continue even if they drop occasional packets. A high-pinger who does not inhibit the overall performance, just his own, is sometimes known as a High Ping Bait.
Low Ping Bastard or LPB is a phrase that originated during the time when Quake was a highly popular multiplayer online first-person shooter. The term came into common usage when broadband Internet connections first came into the reach of home users and were competing with people limited to a 56 kbit/s or slower modem connection. This term was so widely used because of the havoc wreaked upon the other users of these servers by the LPBs due to their overwhelming advantage in reaction time afforded by their connection.
A 56ker is a slang term for a person using a 56 kilobit modem connection. These connections are considered to be slow compared to modern broadband connections and generally result in lewd complaints from other higher bandwidth participants in the game or channel.
Lag can be used, surprisingly, in modding. Some mods are factually designed around lagging all the users except the modder himself, but actually sometimes are caused by accident when a modder makes something spawn out of a weapon, and somebody uses that weapon a lot and spawns loads, this normally lags the modder and forces him/her to restart his/her Xbox as it crashes.
In many cases, lag can be reduced. For Xbox players, just unplug your modem for 30 seconds, plug it back in and then reconnect. For PC users, you can do the same and a lot more. You can change your TCP/IP settings. Since some lag is caused by the system itself, you can perform system maintenance regularly or use commercial software for thorough maintenance. If you are using the same modem and internet connection for your Xbox and PC, and both of them are running at the same time, you can set all the PC's application's bandwidth allocation to the least, or to "co-operative".
People using wireless connections should use relatively good and reliable routers with a good wireless reach distance. Wireless connections can cause a lot of lag if there is not a clear path from the wireless router to the Xbox. This also includes making sure there is no large pieces of metal that could potentially form a barrier for the wireless signal.