The older waiter has wisdom and experience whereas the younger waiter is selfish, inexperienced and dismissive of the old man. The older waiter understands that the old man needs this simple and seemingly trivial routine of sitting in a "clean, well lighted place." The old man is very sad, to the point of being suicidal. He is drowning his sorrows in alcohol, but he gets some solace and peace in being in a quiet, warm, public place.
To emphasize the peace the old man gets from the cafe, Hemingway emphasizes qualities of light, quietness, and cleanliness. In the opening paragraph, the old man sits in "the shadow the leaves of the tree made against the electric light." The shadow illustrates the old man's sadness, but the light is part of what the old man and the old waiter experience as the welcoming ambiance of the cafe. The old man also likes the cafe at this time of night because it is quiet.
The old waiter notes how the old man is "clean" and never spills his drink. The old man respects what the cafe gives him: light, a sense of cleanliness, and a sense of order. Thus, he keeps his area on the table clean as well. The younger waiter has no respect for him and spills his drink while pouring it.
The older waiter adds that he is like the old man. He is lonely as well and he sympathizes with people who "need a light for the night." Light is the most important and repeated symbol in the story. It represents hope. Literally and figuratively, for the old man and the old waiter, the cafe is a "light in the night." That is to say it is a gleam of hope in a dark, meaningless existence.
In explaining how the cafe is different from a bodega, the older waiter repeats the qualities of light and cleanliness. Being clean connotes a sense of order, purity, innocence, honesty, and goodness. As he shuts down the cafe, the old waiter says to himself, "It is the light of course but it is necessary that the place be clean and pleasant." For sad and lonely people, the cafe is a light in the night as well as cleanliness or order in the chaos of life.
The other word/concept that is repeated many times is the idea of nothing or nada. This is most obvious at the end of the story. The notion of nothing is what lies at the heart of the old man's sadness as well as the old waiter's own melancholy. The old man has nothing meaningful left in his life. In his sad, empty life, the only thing that makes him forget his nothingness is the solace he gets from the nightly experience at the cafe. Just as it is a "light in the night," it is also a 'something in the nothing.' It is a quiet, clean, bright feeling of peace in an otherwise empty life. Hemingway uses the dramatic notion of nothingness to emphasize the degree of the old man's sadness. The light of the cafe is a beacon of comfort to him.