Kevin DeYoung quotes Gary Dorrien, a liberal theologian on what liberal theology is.
Fundamentally it is the idea of a genuine Christianity not based on external authority. Liberal theology seeks to re-interpret the symbols of Christianity in a way that creates a progressive religious alternative to atheistic rationalism and to theologies based on external authority. Specifically, liberal theology is defined by its openness to the verdicts of modern intellectual inquiry, especially the natural and social sciences; its commitment to the authority of individual reason and experience; its conception of Christianity as an ethical way of life; its favoring of moral concepts of atonement; and its commitment to make Christianity credible and socially relevant to modern people.
Essentially, the argument is that science, reason, and experience trump the Scriptures.