DDT(a toxic pesticide) shows biological magnification. What does that mean?

It accumulates in the flesh of predators of treated pest organisms. But DDT has very low toxicity to mammals......

Some organic compounds, when released into the environment, fail to break down. Instead, they work their way into the bodies of creatures, which in turn get eaten by other creatures, on and on up the food chain. At each step up the chain, these organic compounds become more and more concentrated.

 For example, DDT is used to treat some farm crops and the residue gets washed into a local stream or river by the rain. Plankton and other microorganisms absorb a little bit of this DDT which now stays in their body forever because they have no mechanism to excrete it out. Now fish eat these microorganisms. And because it takes a lot of these tiny creatures to feed a fish, the fish get a more concentrated dose of the DDT absorbed into their bodies, which stays there too. It tends to migrate into the fat cells, because DDT is organic and it is more partial to dissolve in lipids. These contaminated fish are then eaten by creatures higher up the food chain, such as bears, birds, humans, etc, which eat lots of fish, have no way to get rid of the DDT and therefore build up even greater concentrations of DDT in their fat cells. One contaminated fish is not a big problem, but when your diet consists of lots of slightly contaminated creatures, the end result is that the apex animals in the food chain ultimately get a large concentrated dose of the contaminant. That's the "magnification" effect referred to in biological magnification.