Symbols in Macbeth
Ravens are typically symbols of death or bad omens, creating an ominous atmosphere in a story. Lady Macbeth suggests that the raven's voice is harsh or rough from croaking over the dead bodies on the battlefields, and that it will have reason to croak again at Macbeth's castle with Duncan's fatal arrival.
The Porter, likely still drunk, is imagining what being the gatekeeper of hell would be like and who he would meet. He imagines encountering a farmer who stockpiled his crops to sell at inflated rates during the next famine and then hanged himself when the famine never came. This imaginary farmer represents the sins of greed and suicide, which the Porter thinks would make him a perfect candidate to meet in hell.