Metaphor
A conceptual bridge in which one visual form is used to carry the meaning of another.
In practice
A visual metaphor uses one form to carry the meaning of another, so viewers transfer qualities from the first to the second. A folder on a computer desktop borrows the idea of a paper folder; an advertisement showing a car as a running cheetah suggests speed. The risks are a metaphor the audience does not share, or one so familiar that it no longer says anything new.
Often confused with Symbol
A symbol stands for a concept by fixed convention; a metaphor creates meaning by asking viewers to see one thing in terms of another.
Try this
Pick an abstract idea, such as patience, and sketch three objects that could carry its meaning on a poster.