Descriptive indicates the degree of a given adjective like, "這件襯衫很白。" as opposed to "不白", or "不很白". These descriptions tell us the degree of whiteness.
Delineating varying degrees differs subtlety from categorizing an object, for example, "這件襯衫是白的。" as opposed the entirely different categories of let's say "黑的", "藍的", or "紅的".