Answer:
An allusion is a figure of speech in which something is evoked overtly or implicitly through unrelated images or references that suggest the connection. It is up to the reader or listener to make the connection. It is important in literature in order to add depth to the narrative and dimensions that enrich a writer´s style.
Examples: your love for her will unlock Pandora´s box (the allusion is to the Greek myth of Pandora and its negative consequences).
Or: My friend Bernard is no Newton (that is to say, my friend is no genius, like Isaac Newton was).