The Accumulation/Distribution is really a variation of the more popular 
On Balance Volume indicator.  Both of these indicators 
attempt to confirm changes in prices by comparing the volume associated with prices.
When the Accumulation/Distribution moves up, it shows that the security is being 
accumulated as most of the volume is associated with upward price movement.  When the 
indicator moves down, it shows that the security is being distributed as most of the volume 
is associated with downward price movement.
Divergences between the 
Accumulation/Distribution and the security's price imply a change is imminent.  When a 
divergence does occur, prices usually change to confirm the Accumulation/Distribution.  For 
example, if the indicator is moving up and the security's price is going down, prices will 
probably reverse.