It's about the narcotics industry, paralleling the decay of an urban city, shown through the failing institutions of ghettos/streets, the ports/unions, the political system/city hall, the education system, and the media.
That's the quick overview.
It's a lot more than that though. Perhaps the greatest series I've ever seen. It is truly an amazing show.
Warning-- it is a slow moving and, honestly, quite depressing drama...but captivating and enticing nonetheless. If you don't have much patience or an ability to hold your attention throughout the series, then it may not be for you.