All is Vibration.  Energy is equal to Mass.  Mass is equal to Energy.  In essence,
both are a Vibration.  The Physical world which we can sense with our vision, hearing,
touch, etc., is nothing more than a Vibration, in fact, it is nothing but a mass of slow
frequency vibrations.  Higher frequencies produce a more etheric, transparent quality. 
Still higher frequencies result in our so-called "invisible world", what we call the
Spiritual plane of existence.  But even ultra high frequencies like this are still a
frequency, still a Vibration.  No matter how high or low the frequency, All is Vibration.
        
Our physical body is a slow Vibration, and our Spirit or Soul is a much higher
Vibration.  What we call human life may be described as the occurrence of a Spirit being
encased within a body--one collection of high frequency Vibration being embedded
within (Spirit is within) a collection of lower frequency Vibration.

What we call death may be described as the occurrence of a Spirit being released
from within a body--one collection of high frequency Vibration being released from
within the collection of lower frequency Vibration.

It's not too technical in description and is totally understandable.  You, a human
being, are a Spirit or Soul temporarily residing inside a physical body.  It doesn't get
any simpler than that.

At the moment of "death", your Spirit (which is what you really are) is finally
released from your body (your temporary Spirit-carrier, or shell) once the mechanical
process of breathing stops.  The Science of Yoga teaches that breath is the function
which keeps the Spirit inside the body.  Breath is the tie which binds us to our body. 
When the breath stops, one experiences what we call death, and the Spirit is released.