Macbeth has heard that his wife died but does not realize this as important and meaningless as life is only dust. Tomorrow is a phrase of beats of time that brings with it doom. The candle is maybe the imagery of a soul in which Macbeth's future is grim.
To provide an example for why a book was banned.
The poem is stating, to stop and see what you have, take stock, before you neglect or lose “it”.
“A bird in the hand is worth more than two in a bush”!
