How can I understand the sentence in quotes?
You know enough about the world to know that one way grownups keep track of numbers is to keep score—like in baseball or golf, or "whether your mom or dad last cleaned up after the dog in the back yard".

Thanks a lot.

Question

How can I understand the sentence in quotes?
You know enough about the world to know that one way grownups keep track of numbers is to keep score—like in baseball or golf, or "whether your mom or dad last cleaned up after the dog in the back yard".

Thanks a lot.

Answer

Basically, it means 'keep track of'.

Answer

Meaning that if the mom and dad are the only two who have to do the nasty job of picking up dog poop in the yard that they may say things like "I cleaned it up last time; it's your turn".  People keep score of who did a chore last.