This term has moved from a very precise and technical definition to broader uses. The starting point is narrowcasting as a concept opposed to broadcasting, that is, the signal is transmitted to a limited number of recipients. For example, television networks that are set up so that only subscribers can receive the signal rely on narrowcasting.
From there we move to a general marketing concept, that advertising is directed to the specific groups of people who may be receptive to it. This is in opposition to the concept of mass marketing, the marketers believing that the day of the mass market is over and that we now have a conglomerate of niche markets.
Finally it has become a term in politics for the transmission of political messages to the particular groups in society that will respond well to them. It is important not to get the messages mixed because one group may respond well but another may be outraged. It is a bit like dog-whistling in that sense.