An allusion is a figure of speech. It refers to a well-known story, event, person, or object in order to make a comparison in a person's mind. For example, telling someone "You have Herculean strength!" can refer to Hercules, known as one of the strongest men in Greek Mythology. Others common include:
Allusions can be hurtful, humorous, or easily understandable/relatable. Try for yourself to make an allusion, and people will understand you more easily.