The title of Falling Man works on several levels. It refers to the main character, Keith, who survives the terrorist destruction of the World Trade Center but feels disconnected from the world. Even though he reconciles with his wife and had an intimate connection with another survivor, he feels out of touch with the world, as if he was meant to die but thwarted fate by accident. In this sense, although he was one of the "lucky ones," Keith is still "falling" from his ordeal.
It also refers to the famous photograph of The Falling Man taken by Richard Drew, which shows an unidentified man falling head-first from the burning Towers. This photograph became an iconic representation of the pain and suffering brought about by the attack. In the novel, a performance artist takes up the name and hangs from various objects, recreating the pose to elicit reactions from passersby; in particular, he hangs beside a moving train, on which the passengers cannot see his harness and simply see, in the blink of an eye, a man falling.
All his falls were head first, none announced in advance. The performance pieces were not designed to be recorded by a photographer. Those pictures that exist were taken by people who happened to be at the site...
(DeLillo, Falling Man, Google Books)
The performance artist dies "of natural causes," and it is thought that he meant to fall one last time, but without a safety harness. His reaction to the attack is one of many, and he leaves a lasting impression on Keith's wife, Lianne. Interestingly, DeLillo claims that he named the novel Falling Man without knowing that it was the actual title of the famous photograph.