on Writers Helping Writers:

We all want our writing to be layered. Like a gourmet meal, we want there to be more to them than just what’s seen on the surface.
In stories, this depth can be added a number of ways—through subplots, character arc, subtext, theme, and symbolism.
Of them all, I think symbolism is one of the simplest methods to employ, and it packs a serious wallop.