When it came to concealing his troubles, Tommy Wilhelm was not less capable than the next fellow. So at least he thought and there was a certain amount of evidence to back him up. He had once been an actor—no, not quite, an extra—and he knew what acting should be.
These are the opening lines of the novel, spoken by the third person, omniscient narrator. They establish certain thematic threads that will be woven throughout the book and that are, from the beginning, pointed out as important. First of all, the quote introduces the idea of concealment. It implies that not only does Tommy Wilhelm "keep up appearances," but so does everyone else. In other words it points to the masks and the many layers of the "modern man." This introduction to the idea of "appearances" is then followed, appropriately, by a conversation between Tommy and Rubin—the man from the newspaper stand in the hotel where Tommy is staying—about clothing which is an ultimate symbol of "layering" and "appearances."
Furthermore, it tells the reader that the protagonist was once an actor. However, significantly it also points out that he was not quite so, for he was an extra. This indicates that Tommy may be, in many ways, a failure. Also, it illustrates that perhaps Tommy, who could not make it as an actor, will not be as good as "he thought" at keeping his mask strapped on tightly. In short, it concisely foreshadows the book and gives the reader an idea of who Tommy is all within the first three sentences of the novel.