A Gauss is a common unit of measurement of magnetic field strength. A Gauss meter is an instrument which measures the strength of magnetic fields. Inside a Gauss meter there is a coil of thin wire, typically with hundreds of turns. As a magnetic field radiates through the coil, it induces a current, which is amplified by the circuitry inside the Gauss meter. Gauss meters may vary in the strength of the magnetic field they are capable of measuring.
Gauss meters vary widely in price and accuracy.
Meters have either a single axis coil or a triple axis coil. Single axis meters are much simpler than triple axis meters to manufacture and thus, are less expensive. Then, you combine the three readings in a mathematical equation to calculate the combined field strength. Obviously, its far easier and more accurate to use a 3-axis meter. Triple axis Gauss meters are quite accurate, but they are also more expensive.