I'm starting with one of the most obvious tropes of all: Aliens. Other intelligent species are at the heart of at least half of all sci-fi and a good bunch of mystery stories. Aliens are fascinating for us because they represent both, a threat and great opportunities.

The aliens in the stories almost never represent both. They are either enemies wanting to kill humanity - most of the time for the resources on our planet. Or they are friendly allies.

Aliens are fascinating because their existence would give us the opportunity to extend our knowledge quickly at a speed where classic science would take centuries. This is the perfect vessel for another sci-fi trope: dealing with new knowledge.

But the main trope aliens are used for is the mirror. Aliens mirror human stereotypes to the extreme, so a story can focus on them through a sociological microscope.

From pure rational Vulcans to the Overlords in Arthur C. Clarke's "Childhood's End", all of them tell us something about ourselves.