To begin with, Bioaccumulation means an increase in the concentration of a chemical over time. Why this has an impact on organisms is very important because it affects not just humans but plants and animals. It could have severe damages which could lead to a decline in population of birds to illnesses and deaths in humans. Bioaccumulation in itself is not a bad thing as we as humans need to accumulate nutrients, vitamins and  other things that contribute to our daily health. However there are things we accumulate that is not good for health, the accumulation of toxins and other disease causing illnesses can be a hazard and could be fatal. What toxicologists are concerned about is the amount of toxins we accumulate. 

Bioaccumulation begins when a chemical has entered the cell of an organism. From there on when an animal that consumes the animal containing DDT, the animal that ate the contaminated animal becomes contaminated themselves. As this repeats, the concentration of DDT increases as it is stored on fat tissue of the organism that consumed the DDT containing animal.

The diagram on the right shows how energy flows through a food chain. The figure shows how much calories an organism produces starting with the plant. Also the diagram shows how much calories is left after respiration.  

The pattern is that as you go to a higher trophic level the amount of Calories after respiration reduces significantly.