A specimen is a particular requirement of the USPTO, where the applicant has to prove the active status of the trademark on the US market. It is real-life evidence that the trademark is used in commerce. It shows how the trademark is presented in the marketplace with the goods or services in the application or registration maintenance filing. It is essential to show what customers see when deciding whether to buy a product or use a service, hence the visibility of the trademark.
Regarding physical goods, a specimen demonstrates how a trademark is used in trade with the existing products. A product displaying the trademark on the packaging, a label or tag attached to the goods that bear the brand, or a website where the goods may be ordered or purchased all qualify as specimens.
A specimen for services must demonstrate how the trademark has been used in connection with the current services in commerce in a way that directly links the trademark to the services. Examples include advertisements, brochures, website printouts or other promotional material, as well as business signs displayed at the location where the services are provided or even on a service vehicle. Generally, there is no need for a physical specimen, and all documents (such as screenshots, scanned copies, or photographs) should be sent electronically to the USPTO.
You can learn more about specimen in our Trademark Academy article on intent-to-use vs use-in-commerce applications.