Influenced by Chinese philosophy and Buddhism, Vietnamese cuisine is rooted in five cardinal flavors, each tied to one of the five natural elements: spicy represents metal, sour represents wood, bitter corresponds to fire, salty is water, and sweet is earth. Every dish, or at least every meal, should strike a balance of those flavors and elements.
If this is your first time ordering Pho, see a helpful guide (with pictures) on Pho Meats Guide.