A more thorough explanation:

A declaration under § 15 is a statement made by the owner of a registered trademark. This statement is made after five years of registration and confirms that the mark has been in continuous use in commerce for at least five consecutive years since registration. The declaration also confirms that the mark has not become generic, that there has been no final adverse decision to ownership in the mark, and that there is no pending proceeding in the U.S. Patent and Trademark Office or courts involving the mark.

The purpose of this statement is to provide the mark with immunity from some legal challenges under § 15 of the Lanham Act. This means that the mark cannot be challenged on certain grounds, such as its validity or ownership, after the declaration has been made.

For example, if a company has registered a trademark for a specific product and has been using that trademark for five years without any legal challenges, they can make a declaration under § 15. This declaration will protect their trademark from future challenges on certain grounds.