b = map(a, min1, max1, min2, max2);

takes a value of a in the range (min1, max1), and then calculates where in the range min2, max2 it falls....

So map(5, 0, 10, 20, 40) should return 30..... just saves you writing your own conversion equations when for example, you want to get a value from 0 to width (e.g. mouseX) and map it to the values 0-255 (alpha, say).

alpha = map(mouseX, 0, width, 0, 255);