Definition:
A symbol is literary device that contains several layers of meaning, often concealed at first sight, and is representative of several other aspects, concepts or traits than those that are visible in the literal translation alone. Symbol is using an object or action that means something more than its literal meaning.
Example:
The phrase “a new dawn” does not talk only about the actual beginning of a new day but also signifies a new start, a fresh chance to begin and the end of a previous tiring time.
Harry Potter
Symbols can also foreshadow something in a piece of literature such as:
The Grim–death or terrible/tragic misfortunes
The Grim in Harry’s cup foreshadowed the fights w/ Voldemort, the death eaters, loss of friends, etc. but it also foreshadowed Sirius Black.
Sirius could transform into a black wolf that looked highly similar to the grim.