The show deals with many of Baltimore's city (Washington, USA) problems. It is related to the notion of power in many ways. First of all, and from my point of view, the power of gangs.
Baltimore city is a pretty chaotic place to live in. It has a wide number of african american population, but most of it lives in the most dangerous and troubled neighbourhoods of Baltimore. Indeed, the gangs are in their territory, and they use these neighbourhoods as "marketplaces" for their drug dealing and other illegal activities. The series shows how powerful can gangs be, even if they are in one if the most developed countries in the world: The USA. All through the episodes we can see how the police are powerless before the gangs' actions. Indeed, there is no easy way of stopping them. They enjoy of a great influence in the political spheres, allowing them to get away from many things. Also, a lot of their gangsters start dealing drugs and running errands for them at a very young age, which makes it very hard to catch them for two main reasons: one of them being the fact that since they are mere children they can easily hide in crowds and the other being that children cannot be prosecuted for their crimes until they're16/17 years old (depending on the crime).
The show also reveals the power of drugs, in a very disturbing way. During the show, we see different caracters fall into misery because of them. Indeed, it conveys a sad reality: teenagers living in that kind of neighbourhoods need to face the only reality they live: the drug world. Some of them decide to become drug dealers, employed by rich and powerful gansgters, and accepting that they will die sooner rather than later. On the other hand, some of them, with no parents to take care of their child, fall gradually into drugs, misery and addiction. One of the main characters of the show, a young boy no older than any high schooler, bright, merciful and brilliant, ends up being addicted to drugs, sharing needles with homeless people and living under bridges or such. Unfortunately, drugs do have the power to destroy a person, even a teenager, and lead them into a dark world, from which it is almost impossible to escape.
I personally believe that the most implicit form of power is the power of money. Indeed, one of the policeman shown in the trailer states it: "Follow the money, and you will have everything". The series shows that money drives everything: drug dealers murder and create life dramas for the money, politicians look the other way or make personal favours for the money, and ultimately, it's money what stops the police corps of being more efficient. At one point of the show, the police corps experience budget cutbacks, and since they cannot repair the cars, or pay for the extra hours they would need to investigate on the drug dealers, they are powerless before the cruel war going on between gangs in their own city.
On the whole, the shows deals with many forms of power, mainly the power of gangs, drugs and finally money. It conveys the sad reality we live in, and that unfortunately, much like drugs, seems to not have a way out.