This line is from Scene Four in the first act of Macbeth, and essentially Macbeth is saying (to himself, the line is an aside) that he does not want the world to know of the terrible things that he is beginning to plot. He has just learned that Malcolm, Duncan's son, has been named Prince of Cumberland, thus making him heir to the throne upon Duncan's death. Immediately before uttering the line in question he says to himself:
The Prince of Cumberland! That is a step
On which I must fall down, or else o'erleap...
It is becoming obvious that Macbeth, having been informed by the witches that he is to be Thane of Cawdor and King of Scotland, is not willing to wait to let that happen. But he does not want anyone to know of his ambition, and wishes to be seen as the loyal vassal of Duncan. This, of course, is for practical reasons, as he obviously wouldn't want to reveal his plot, but he is also clearly torn between his sense of right and wrong and his considerable ambition. For these reasons, he does not want his thoughts to see the light of day.