What does small r mean in Rubik's Cube?

Related questions

1. How long would it take to solve a Rubik's Cube randomly?
2. How hard is it to solve Rubik's Cube?
3. What's the fastest Rubik's Cube time?
4. How many different Rubik's Cube combinations are there?
5. How long did it take Erno Rubik to solve the Rubik's Cube?
6. What does Rubik mean?
7. What does y2 mean in cubing?
8. Is Rubik's Cube good for your brain?
9. How many turns does it take to solve a Rubik's Cube?
10. What does M mean in cubing?

Tags

1. #rubik
2. #algorithm
3. #prime
4. #counterclockwise
5. #front
6. #right
7. #means