Ping is the time between when you throw your ball against a wall and when you receive the ball...
It's the time, when your computer send data to another computer and receive data from the computer.
If your ping is 200, it's take 200 millisecond to send and receive data.
In Medal Of Honor, Wolfenstein, Warcraft 3 I Can play lagless with a ping of 250.. but 250 it's my MAXIMUM PING to play. But in Civ3:PTW.... better have a ping of <150.
have a nice day