A more thorough explanation:

A declaration of use is a statement made by the owner of a registered trademark, confirming that the trademark is currently being used in commerce. The owner must also provide a sample or image of the trademark being used. This statement is required to be filed with the U.S. Patent and Trademark Office every five to ten years after the initial registration of the trademark.

If the owner fails to file this statement within the required time, the registration of the trademark may be cancelled.

Let's say a company registered the trademark "ABC" for their line of clothing. Five years after the initial registration, they must file a declaration of use with the U.S. Patent and Trademark Office, confirming that they are still using the "ABC" trademark on their clothing and providing a sample of the trademark being used.

If they fail to file this statement, their trademark registration may be cancelled, and they may lose the legal protection that comes with having a registered trademark.