Something is metaphorical when you use it to stand for, or symbolize, another thing. For example, a dark sky in a poem might be a metaphorical representation of sadness.
You'll find yourself using the adjective metaphorical all the time if you take a poetry class; poems are usually full of metaphors. People use metaphorical language in everyday speech too, like when you say something like, "My brain is foggy today." Metaphorical comes from the Greek word meaning "a transfer," metaphora, which is itself made up of meta, "over," and pherein, "to carry."