Macbeth reacts to the news that his wife is dead by feeling that existence is meaningless. Everything he, or anyone else does is meaningless.
He compares life to an actor's brief career on stage. An actor is famous for a short while and then disappears from the stage and is never heard from again. All is meaningless.
Macbeth's initial reaction to the bad news is that there would have been a better time for his wife to die--later on, when she was older and death expected. The play's present is not the time for her to die. Her death is premature.
He moves from this thought into the idea that nothing anyone does amounts to anything anyway. His nihilism is expressed through the theatrical metaphor, as well as the statement that humans are just "lighted fools" and all of our actions lead only to death.