Marks that are descriptive or generic are weak. Weak trademarks are often not federally registerable because they fail to clearly indicate the source of the goods or services. Descriptive marks are marks that merely describe an aspect of the goods and services without actually distinguishing the source, such as “crunchy” for cereal. Generic marks are not even trademarks; they are the common name for the goods and services used in everyday conversations. An example would be “water bottle” for a water bottle. Weak trademarks are difficult to protect and defend against competitors, while a strong trademark that avoids generic or descriptive language will provide broader legal protections.
Source: https://www.uspto.gov/trademarks/basics/strong-trademarks