A luminous object is one that gives off light. In other words, it glows (or shines).
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. Stars shine using energy created by nuclear fusion in their cores. Both a torch and a star are considered to be luminous objects.
If an object is not luminous, we can only see it when it reflects light from somewhere else. Most of the objects we see, such as cars, clouds or even the Moon, are not luminous; it is just that they reflect light from the Sun.