Question:  What is light? It has been described to me as radiant energy.  
It would seem to me that light has to be some type of physical substance flowing
between the sources and wherever the light goes.  What is it ?
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Answer:  There are several ways to think of light.  The classical description
says light is an electro-magnetic wave.  This means that it is a varying
electric and magnetic field, which spreads out or propagates from one
place to another.  It is not a physical substance.  The modern quantum
mechanical description says that light can also be considered to be
particles called photons.  These carry energy and momentum but have no mass.
In both descriptions, the light energy is carried by a very real and
observable mechanism.  But I would not call it a physical substance.