Agents of socialization affect political behavior because they teach us what sorts of political beliefs we are expected to have and they teach us how salient politics is to people like us.
The agents of socialization are the people who teach us what is expected of us in our society. Among the most important of these agents are our parents and our friends. In terms of politics, we may also be socialized by teachers and even by the media.
These agents tell us what political attitudes people like us have. They help to tell us whether we will be conservative or liberal. They also help to tell us how involved in politics we will be. For example if you grow up in a household where politics is never discussed and you never even see your parents vote, you will likely not participate much in politics. This is because you have been socialized to believe that it is not important. By contrast, if your parents or members of your church, or other people close to you are constantly talking about politics and are involved in political campaigns and other such things, you are much more likely to participate yourself.
Thus, agents of socialization help us to understand what sorts of beliefs people like us have and how we act on those beliefs. This has a tremendous impact on our political behavior.