Material is an asset that can be applied to a mesh during rendering to define the visual look of the object. The easiest way to think about a Material is the paint that covers the object surface. But that's not all because each individual material defines various surface properties like color, roughness, metalness and more. All those properties enable artists to define desire scene look.
Speaking more technically, when light from the scene hits the surface, a Material is used to calculate how light interacts with that surface. These calculations are done using all the material surface properties. This section covers all basics when using materials as well as more advanced topics.