Magnetometer values

Hello,

I am using the CA-SDK board, but I don't understand the magnetometer values. When I don’t move the board the output is: -2.5665 -6.2953 43.1705. When I’m rotating the board about 90° around the Z-axis the output is -2.9872 -5.8453 41.5206. Then I rotate again over an angle of 90° (180° relative to the starting position) the output is: -3.1554 -6.5953 41.2206.
How do I know the position of North, East, South and West is? What is the meaning of these values?

Thanks in advance.