Ping is computer terminology for the time it takes bits information to be “pinged” back and forth from one computer to another. Technically, it is a command used to measure that time, and that time is called latency, but since games usually report the result of the ping test, and don't force people to run the operation themselves, most fraggers use the terms interchangeably. This goes back to when DARPA was first developed and really smart computer guys (nerds, okay?) would send a ping to test to see how long the other computer would take to respond—then the connection sends a “pong” back, and they measure that span of time in fractions of a second. If computers are communicating fast and seamlessly, this gives you a low or good ping. Having “low ping” may sound like a bad thing, but trust me it really isn’t. It measures how long it takes what you are doing to get to the server in milliseconds, so the lower the better. Anything under a hundred is good. (To the right is a list of Internet servers for Halo and their respective pings--the blue arrow is pointing to ping column. Most online FPS video game servers have a similar lobby to browse online matches--COD, Halo, Ghost Recon, etc.)
If you are slightly more advanced or technically inclined you can get in there and check your network settings at the back end with various command prompts or administrative tools. Most games have some sort of diagnostic mode where you can take a look at how fast your connection speed is and what your settings are. Remember, it's all about optimizing your levels to match your hardware, software and ISP resources.