When do we use double quotes for table names in SQL?

Question

A SQL coding challenge provided a database; every table was accessible when passed as a string using double quotes and not when passed as a word as I am normally used to.

This did not work:

SELECT * FROM Athletes;

Error message: relation does not exist.

But this worked and I don't understand why:

SELECT * FROM "Athletes";

Was this defined during the database creation? Or is this from PostgreSQL?