True Vegetarians Make Dull Company
When you plot your course for a trip the above helps you remember how to adjust for all the magnetic and compass variations.
True Course +/- Magnetic Variation = Magnetic Heading +/- Magnetic Deviation = Course to fly
In preparation for a trip you would first you will draw a line from point A to point B on your sectional - that is your True Course. Since the sectional is printed with reference to True North you will have to adjust this value so that is referenced to Magnetic North (since you will use your compass and Heading Indicator ). The sectional has the isogonic lines or Magnetic Variation printed on it with the magenta dashed lines and the value. Adding or subtracting this number to your true course will give you your Magnetic Heading. But... due to installation and magnetic forces in the plane there are teeny tiny corrections that need to made for that. You can find the corrections on a card that is under your compass, it is normally a correction of +/- 1 degree, adding this or subtracting that value will give you the actual course you will fly.
What I didn't discuss is wind because that will adjust your course again but this is an entry taking about Isogonic Lines