What does Macbeth mean by his "Tomorrow, and tomorrow, and tomorrow" speech?

In this speech, Macbeth is so low that he is simply resigned to what he has just been told: his wife has died, but his first comment on the matter is that "she should have died hereafter"—that is, she would have died at some point anyway. Then, however, he goes on to lament the fact that time seems to "creep" on from one tomorrow to the next, inexorably and yet with seeming monotonous slowness without anything ever really changing. "All our yesterdays," he says—all the days which seem so important to us—are really just a procession of moments in our march towards death. We are all, in the end, "fools" who care about their lives without thinking about how fragile they are.
Macbeth describes human lives as like a "brief candle," no sooner lit than snuffed out. He can see no hope in living anymore, but is almost beyond trying to do anything about it. Life seems like a "shadow" to him, with each person a mere "player" on a stage who is only there long enough to play his turn. Ultimately, while life may be full of huge ups and downs for those living it—"sound and fury"—it actually means nothing and has no ultimate impact on the ongoing passage of time.

Tomorrow, and tomorrow, and tomorrow Creeps in this petty pace from day to day To the last syllable of recorded time; And all our yesterdays have lighted fools The way to dusty death.
These beautiful lines are especially impressive because of the contrast between "Tomorrow and tomorrow and tomorrow," and "all our yesterdays." The word "tomorrow" is looking forward to the future. We are always doing this. If today hasn't brought the good thing we were hoping for, then maybe it will come tomorrow. If Godot didn't come today, maybe he will come tomorrow. Each successive tomorrow becomes a yesterday. Tomorrows represent hope; yesterdays represent memory. The number of tomorrows dwindles as we age. The number of yesterdays piles up as we get older and seem to extend back into a dark infinity. These are the musings of a depressed and thoroughly disappointed man. In looking back on all his "yesterdays," he can't help thinking that "life is but a walking shadow." Every person who appears in our memories, as in our dreams, looks like a shadow or a ghost of the person we knew. Macbeth evidently does not feel much of anything about his wife's demise because he regards her as just another walking shadow like himself.