A more thorough explanation:

A Section 15 Declaration is a statement made by the owner of a registered trademark after five years of registration. The statement confirms that the trademark has been in continuous use in commerce for at least five consecutive years since registration, has not become generic, and there is no pending proceeding in the U.S. Patent and Trademark Office or courts involving the mark. This declaration entitles the mark to immunity from some legal challenges under § 15 of the Lanham Act.

For example, if a company registered a trademark for their product and has been using it for five years without any legal challenges, they can submit a Section 15 Declaration to protect their trademark from future challenges.