A more thorough explanation:

A descriptive mark is a type of trademark that uses words or phrases that describe the characteristics or qualities of the goods or services associated with the mark. Unlike suggestive marks, which require consumers to use reasoning to understand the mark's significance, descriptive marks receive less protection under the Lanham Act because they directly illustrate some characteristic of the products that require very little explanation to understand their meaning.

For example, "Apple" is a descriptive mark for a company that sells apples. "Best Buy" is a descriptive mark for a store that sells electronics. These marks are not inherently distinctive because they describe the products or services being offered.

However, descriptive marks can come under protection if they acquire distinctiveness related to the seller's goods or services. This means that over time, consumers come to associate the mark with a particular source of goods or services, rather than just the descriptive meaning of the mark itself.

Courts determine whether a mark is merely descriptive by looking at the context in which the goods or services are used and their significance to an average consumer. In the Federal Circuit, this is a question of fact, and the party trying to prevent the registration of the mark bears the burden of proof that a mark is descriptive by a preponderance of the evidence.

For example, "Holiday Inn" was initially considered a descriptive mark for a chain of hotels. However, over time, the mark acquired distinctiveness as consumers came to associate it with a particular source of hotel services.

In the Ninth Circuit, the courts generally apply the imagination test and the competitors' needs test to distinguish merely descriptive marks from suggestive marks. The imagination test asks whether a consumer must use one's imagination to understand the nature of the product. The competitors' needs test examines the extent to which the competitors need the mark for their products. If the competitors have a high need to use the mark, the mark is probably descriptive.

Overall, descriptive marks are less distinctive than other types of trademarks, but they can still be protected if they acquire distinctiveness over time.