The Fresnel term results from a complete analysis of the reflection process
while considering light as an electromagnetic wave.
The electric
field of light has a magnetic field associated with it (hence the name electromagnetic).
The magnetic field is always orthogonal to the electric field and the direction
of propagation. Over time the orientation of the electric field may rotate.
If the electric field is oriented in a particular constant direction it is called
polarized.
The behavior of reflection depends on how the incoming electric field is oriented
relative to the surface at the point where the field makes contact.
This variation in reflectance is called the Fresnel effect.
The electric
field of light has a magnetic field associated with it (hence the name electromagnetic).