Symbolism is one way to create layers of meaning within a text. A symbol is a word that has an associated meaning attached to it; for example, a cross can be used to suggest any of several aspects of Christianity such as faith, kindness; a rose can be used to suggest romance or love; in Great Britain, the Union Jack flag can symbolise patriotism and pride.
In literary texts, writers often use symbols - and some of these only become clear as symbols as you read the story. These literary symbols can be quite hard to detect and understand so they gather a high mark when you discuss how they contribute to the text's meaning and themes. In John Steinbeck's story, 'Of Mice and Men', the 'busted back' of Crooks, the downtrodden black character, could be said to symbolise the 'broken backs' of all discriminated black people in the USA of the 1930s; similarly the spurs and belt worn by the 'Boss' symbolise the power he holds over the other ranch workers. In William Golding's novel, 'The Lord of the Flies', fire can be seen as a symbol for power.