In literature, objects have a literal meaning--a rock is a rock, snow is snow, a stormy day is a stormy day. However, the author frequently infuses an object with non-literal meaning. When an author does this s/he is building layers of deeper meaning into the story. This is known as symbolism. Symbolism can be overt or subtle. Sometimes the author will repeatedly make reference or use of the object, or s/he may use variations of the object to convey deeper meaning.
For example: in literature snow is symbolic of death. A story may repeated use snow storms to convey meaning, or the author may use a blizzard, a snow drift, a sleet storm, and etc. to convey the meaning.