Like Balance, structure is an esoteric term. After all, you can't taste a "structure" nor can you taste a "balance." They come from a sense of all the sensory components of a coffee, characterizing the relation between flavors, acids, mouthfeel and aftertaste as well-defined and comprehensive. Well-structured coffee has an architectural feel, as something that is "built", well-founded, solid, with flavors and sapid experiences that relate well to each other. Usually it refers directly to the acidity, or perhaps we might say the acidity is a core component of structure, since a coffee with weak acidity tastes limp and flat.