Allusion

An allusion is an indirect reference to a person, place, or thing. It mainly references past events or figures that the reader are expected to know. Because the author expects the reader to know of these events, the author does not go into much detail about the reference. 
Example: 
“So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.”
-Robert Frost
Robert Frost is referring to the Garden of Eden in the Book of Genesis. The author expects the reader to know about the Garden of Eden as he doesn’t go into detail about the reference. The Garden was a happy place where God could live in unity with his children, but Adam and Eve made a huge mistake by eating the apple from the tree. Now, we are on Earth suffering, and Eden was just a happy memory (the gold that didn’t stay).
https://www.youtube.com/watch?v=8xg3vE8Ie_E
These lyrics from the song “Love Story” by Taylor Swift refer to the play by Shakespeare, Romeo and Juliet, a very famous couple known for their forbidden love.