This is in respect to the ways the three characters deal with uncertainty and what they have to live for.
The old man drinks his sorrows away to forget the void in his life: the “nada.” He has already attempted suicide. He has faced despair and has decided to escape life or carefully drink away his loss in the comfort of the lighted café. The café is his crutch for dealing with his loneliness.
The older waiter understands the old man’s need for the café. And the old waiter accepts the nothing in his own life and says he has nothing except his work. His work is to be with people and provide a clean well-lighted place. The older waiter gets comfort in the solidarity of suffering. Hence, his final line that “many must have” insomnia. He finds comfort in that he is, like the old man, not alone in suffering; not alone in facing the voids (nothings and nadas) in life. He is able to appreciate the light but unafraid to face emptiness and loneliness.
The younger waiter doesn’t get that the light is a beacon in the nothingness (night). The younger waiter is oblivious to concerns that are not his own. The old waiter jokes that the younger waiter wants to get home to make sure his wife is being faithful. The younger waiter’s confidence takes a hit here. The old waiter says to him, “you have youth, confidence and a job.” Someday, he won’t have youth; his confidence is shaken, so the young waiter, although he still doesn’t get it, is (or will be) more like the old man than he thinks.
I think the old man and the young waiter represent opposite poles; the former is in despair, living in the past, drinking to forget what he presently does not have. The latter is just concerned with what he has in the present and, unlike the old man and older waiter, he has not yet experienced loss which is why he cannot commiserate and empathize with the old man as the old waiter does.