A is to B as C is to D.
A and B are related in the same way (and direction) that C and D are related.
If you took the SAT before 2005, you might remember that nasty little analogies section, where you'd have to fill out things like
esoteric word:overly intellectual word::nonsense obscurity:____________
That comparison is an analogy—it's a metaphorical way of evaluating a relationship between two things.