Marlow's statement reveals his beliefs about the existential and political condition of human beings. This condition of human beings that "live in the flicker" helps to convey their temporal predicament. Marlow speaks from a position in which what he has seen is representative of his thoughts. His experiences, sitting as "the Buddha" on the Nellie, is one in which he believes that what we, as human beings do, exist in the brief instant, "the flicker." The "old earth keeps rolling" without our consent, and devoid of the flicker. The flicker, the human beings' endeavors, comes and goes, while the "old earth keeps rolling." Marlow's statement, expressed at the exposition of the novel helps to establish this condition of human beings.
Another understanding of the statement can help to illuminate and almost foreshadow more of the story. The darkness that the flicker seeks to illuminate is permanent. There is no escaping it. This could very well be the "heart of darkness." There is darkness that surrounds "the flicker." All attempts to escape from this condition are futile, for there is only the darkness that envelops and consumes "the flicker." For Marlow, his construction of the human condition is one in which the flicker is temporary. The darkness is permanent.