The mirror simply appears as a tall, wooden mirror with a reflective, dark blue surface and a skull mask feature that matches Death's own on top of the wood.
A few times, it is mentioned that if allowed, some can step through Lord Death's mirror.
This "mirror number" is used to communicate via mirrors or any transparent window, to contact Shinigami's Mirror and call Death. For example, the user will find a mirror and blow air onto it to make it condensed, then they will write the "mirror number" of the person they want to call. For instance, Death's number is "42-42-564".