Intro to CSS Lesson - What is the difference between CSS Rules and Rule Sets

https://www.codecademy.com/paths/learn-how-to-build-websites/tracks/intro-to-css/modules/learn-css-selectors-visual-rules/lessons/css-visual-rules/exercises/css-structure

In this lesson, the author refers to “rules or rule sets”. Can you please explain the difference? or are they just different names for the same thing?

Thank you