The idea of "The Fall," like every important concept in the novel, takes on more than one meaning. We’ll explore three of these meanings here, but you can come up with more.
The most obvious interpretation is the focus of Jean-Baptiste Clamence’s "confession" – his own "fall from grace," so to speak. Jean-Baptiste was living the sweet life in Paris – booze, women, money, success, sex – before he "fell" metaphorically into the seedy life he now has in Amsterdam.
You’ve probably heard the phrase "The Fall" in reference to Adam and Eve, as in the first man and woman ever, according to the Judeo-Christian tradition. In this Biblical scenario, Adam and Eve’s decision to eat an apple becomes The Fall of Man, and marks humanity's metaphorical transition from God’s good graces. Physically, it constitutes their exile from the Garden of Eden. In Camus’s The Fall, Jean-Baptiste’s own fall also makes sense when you view Jean-Baptiste as God himself (see "Character Analysis" for more). Jean-Baptiste sees himself as perfect, then he sees himself as a sinner. If Jean-Baptiste = God, then this new vantage point is very much like Adam’s fall from God’s grace.
The second interpretation has to do with Lucifer, or the Devil. Once upon a time, Lucifer was an angel. God’s best angel, in fact. Then he rebelled against God and was cast out of heaven, at which point he relocated to hell. Permanently. Lucifer got the epithet of "fallen angel," because he fell from Heaven down to hell. One of Jean-Baptiste’s many roles is that of Lucifer. He even refers to Amsterdam as hell.
The third interpretation centers on the woman falling off the bridge and into the river, a.k.a. the moment that constituted Jean-Baptiste’s own fall.