A luminous object is one that gives off light. In other words, it glows (or shines) of its own accord.
To be able to glow, the object must have its own source of energy. For example, a torch shines because of the energy stored in its batteries, whereas all stars shine using energy created by nuclear fusion in their cores. Both a torch and a star are considered to be luminous objects.
In order for us to see an object that is non-luminous, it must reflect some of the light it receives from a luminous source, such as the Sun. Most of the objects we see, such as cars, clouds or even the Moon, are not luminous; it is just that they reflect sunlight.