In literary terms symbols are elements that have a broader reference than their denotation. For example a white dove can be defined as “a variety of the domestic pigeon”. Put that same dove with a laurel twig in its mouth against an azure sky and it becomes a symbol for peace.
Common symbols include the colours “black” and “white” (which make it clear who is the villain and who is the good guy or gal in a mystery movie), thunder and lightning, reflecting a tumultuous mood or atmosphere, and hearts, representing love. Unless, of course, that heart is pierced by an arrow symbolizing betrayal or rejection.
When Macbeth asks Ross and Angus “why [they dress him] in borrowed robes”, the clothing metaphor acts as the symbol of a position that Macbeth is as yet unaware of.
As the play progresses with reference to “strange garments”, the wearing of “old robes [which should not] sit easier than our new”, and “a giant’s robe upon a dwarfish thief”, the symbol becomes a motif: a type of sustained metaphor reflecting aspects of character and adding depth to our understanding of their psychology.