Coordinates (or coords) are numbers determining the location of the player's planet in the universe. Coordinates are given in the form of [x:yyy:zz], where "x" denotes the galaxy, "yyy" the solar system in the galaxy, and "zz" the position of the planet within the solar system.
Players have to remember that giving coords is strictly forbidden on the forums and official IRC channels, although they are of course not forbidden in private talk like PM's or queries.