This concept is related to African Americans because the whole idea of an African American is defined in a way that is based on our societal ideas.
For an example of this, let us look at President Obama. This is a man who is always identified as an African American. Yet this is a social construct. Obama could just as well be called a white man in that he is half white and half black. We choose to call him black as a matter of social convention. This extends even farther to people who clearly have less African blood in them (based on their looks) than President Obama does. The man in this picture is an NBA player named Klay Thompson. He is defined in our society as an African American. Thompson and Obama are both seen as belonging to the same race and both are defined as belonging to the same race as, for example, Justice Clarence Thoms, shown here.
As a society, we have chosen to define all of these people who look rather different as members of the same race. We also define people who have just moved here from Africa as being in the same race as someone whose ancestors have been in North America for 300 years.
In these ways, we can see that the African American race is something that we choose to define in a certain way. This shows the connection between African Americans and the idea that race is a social construct.