An object is visible if it is either reflecting light or a light source itself. Furthermore, a reflecting object can have specular reflection (glossy finish, mirror-like), diffuse reflection (matte), or the combination of both.
Some flat-Earthers like to compare the appearance of the Moon to a glossy, metallic ball. Then, the difference in how the two objects look is taken as “proof” that the Moon is not reflecting light. They are wrong. A shiny metallic ball reflects almost all incoming light in mirror-like fashion, or that it has predominantly specular reflection. On the other hand, the Moon reflects incoming light to every direction, or that it has only the diffuse reflection.