What is Walter Lippmann's point in Public Opinion?

In Public Opinion, Walter Lippmann details what he sees as a major problem with democracy: the difference between the objective, real-life world and the subjective world inside the head of the average American citizen.
Lippmann conceptualizes the interplay between fact and opinion as a "triangular relationship" between three things:
1. The world as it is (the "scene")
2. The interpretation of the "scene" by the average voter (the "picture")
3. The "human response" to the "picture"
As Lippmann sees it, there is a reciprocal relationship between these three constructs. He calls the sum total of all individual, subjective "human responses" the "Public Opinion."
His point, as your question asks, is that the public opinion does not represent objective truth, and that it should be a mutually agreed-upon goal of all members of a democracy to right public opinion toward truth. In his final section, Lippmann lays out his plan to do just that by better utilizing accountability and organizational tools to "bring our public opinions into grip with the environment."