厚厚的 is a reduplicated form of 厚 (thick). Reduplication adds emphasis and vividness — it conveys that something is noticeably thick, plump, or solid, often with a satisfying or positive nuance. As a state word (状态词), it can be used attributively before a noun (厚厚的书) or predicatively (这本书厚厚的). It is commonly used to describe books, coats, layers of snow, blankets, and similar objects.