"To understand the rules" means to have knowledge or comprehension of the guidelines or principles that govern a certain situation or activity. Full definition
As well, in some cases a person with a cognitive impairment may «do it harder» in prison because of difficulty understanding the rules of prison and the experience more generally. (croakey.org)
I'll admit that when I was younger and didn't understand the rules of fashion, I was not a fan of denim jackets. (cheersj.com)
Unfortunately, people who separate don't always understand the rules for reporting income, assets and expenses. (moneysense.ca)