Isogonic Lines- lines of equal magnetic variation
Agonic Line - line where there is no variation
The magnetic compass naturally points to the Magnetic North Pole, which is NOT geographical North. The sectional chart is printed with Geographical North on top, and the Isogonic lines are illustrated with a dashed magenta line. Here where I live the variation from Magnetic and Geographical north is 15 E. Sooooooo when flying with reference to the magnetic compass/DG you have to allow for this when planning your flight course (plus some small variations ie compass position error). If the variation is East then you subtract and if West you add (East is least and West is best).
All of this is important when you are planning a flight but it is also important when reporting your position to ATC. A quick glance at the compass/HI will verify your magnetic position.
http://www.ngdc.noaa.gov/seg/WMM/data/wmm-D05.pdf shows the lines and how the circle the earth. When you see the magenta dashed lines on a sectional it can be difficult to see how it all fits.
Way too much information but something I find interesting is that the earth's poles reverse every 230,00 years.