Symbolism is a technique in literature using symbols to represent a complex idea or concept. A symbol is anything that stands for or represents something else.
Some symbols are common cultural constructs; others may be unique to a story. For instance stormy weather often represents "stormy" relationships. Some cultures have specific superstitions that may be used as symbols. The color red may be seen as lucky, so in some cultures red objects may indicate good luck. In contrast, a book could introduce a toy, plastic, elephant as a cherished object, and the placement and treatment of that toy as the story progresses could represent the development and status of the relationship of its owner with the person who gave the gift.