These notes assume a basic understanding of budget lines and indifference curves. One place to go online for this information is http://en.wikipedia.org/wiki/Indifference_curve. What follows is calculus-free. At times this means that the discussion gets a little tedious but hang in there or skip ahead.
Think about Fred’s indifference curve. The slope of Fred’s indifference curve is the value to Fred of a little more hamburger, measured in terms of french fries. The slope is how much Fred is willing to pay to get a little bit more hamburger. Instead of being measured in money, it is measured in french fries. We will be comparing value and cost and because the true cost of a hamburger to Fred is the french fries he must give up, it is useful to measure the value of a hamburger in french fries also.