"Overall" can mean "total." For example: "Looking at your car insurance, we can give you a discount of 10% because your car isn't a model that is frequently stolen, and we can give you another discount of 10% because you are also insuring your house with us, so overall your discount should be about 20%." The sentence is talking about "the greatest threat." In order to say something is "the greatest threat," we need a way to measure the size of the threat. "in terms of" means "using these ways of measuring the threat." The first way is by the amount of organized criminal involvement, the second is by the illegal proceeds obtained (the amount of money involved). The third way is "the overall harm caused." Drug trafficking causes a number of different kinds of harm. "overall harm" means, in some way, combining the different kinds of harm to come up with a measure of "overall harm."

In this case "overall harm caused" means when you add up all the ways that society can be harmed by different crime 1. health 2. crime against people 3. violence drug-trafficking does the most damage. To say it another way Compared to other types of crime drug-trafficking does the most damage in all categories. Does that make sense?