In the play "Macbeth" by William Shakespeare, it is perhaps depression as well as time that "creeps in this petty pace from day to day" and "lights fools" to a dusty grave. Much has been written by critics about the mental state of Macbeth and analysing which form of psychiatric disorder, if any,he may have been suffereing from. The line about time creeping is symptomatic of the outward expressions of negativism by Clinical Depression sufferers. Another symptom of this type of depression is a strange combination of sleeplessness, yet a terrible sense of time dragging its feet with nothing for the patient to look forward to. Now Macbeth has displayed both sets of symptoms and things look bleaker than ever as he hears of the death of his wife.
Shakespeare's line is a reference to time. The idea of "creeping" as a description of time undercuts all of human activity. While individuals might feel that what is done is valid and worthy of being seen as the center of all consciousness, it is nothing more than a speck of time, ticking away from its moment of being into a moment of what has been. The notion of this pace being "petty" helps to highlight the trivial nature of all human interactions, the idea that what we do has all encompassing meaning. In the final analysis, our actions and our state of being is a transitory one that is subject and victim to the "petty pace" of time. This line puts human interaction into some perspective.
The line you ask about in Shakespeare's Macbeth is from the famous "Tomorrow" speech, spoken by Macbeth. Here's the line with some context:
She should have died hereafter [at some better time; later, rather than sooner];
There would have been a time for such a word [the word he just received that his wife was dead].
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.... (Act 5.5.17-23)
The repetition of tomorrow suggests tedium, endless repetition, and tomorrow creeps in slowly from one day to the next, always, and it is just as useless as all of the yesterdays.
Macbeth slips into nihilism, here, the belief that nothing matters, and nothing makes any difference. His wife's death is the trigger that moves him to hopelessness. All the yesterdays lead one only to the dusty grave. And between the beginning and the end what happens is irrelevant.
Interestingly, this scene is one of several that shows Macbeth figuratively jumping back and forth between emotionally feeling he is invincible because of the witches' predictions, and rationally knowing the predictions are too good to be true and that he is doomed.
Just this line itself? All it means is that it (time) is going very slowly. It is creeping (crawling) at a petty pace (very slowly). It will keep doing this forever.
This line is spoken by Macbeth in Act V, Scene 5 of the play. The situation is that he has just heard that his wife is dead. When he hears this, he reacts very sadly. He says the line you mention, and then he starts talking about how pointless life is. He says that it is a tale told by a fool and that it means nothing at all.