knowing which side his bread was butter on, he called them the Medicean stars

first there is a grammatical mistake.
"butter" is wrong, it should be "buttered".

knowing which side his bread was buttered on, he called them the Medicean stars.

知道自己的利害所在之后，他称他们为美第奇家族之星。

注意：是“利害”不是“厉害”

bread指「面包」，butter在此当动词，指「抹上黄油」。西方人吃面包时习惯在一面抹上黄油，当然有黄油的那一面比较好吃，所以know which side one's bread is buttered on就是「知道面包哪一面抹了奶油」。引申为「知道什么对自己最有利」之意。

知道了他在面包的哪一侧抹了黄油之后，他称他们为美第奇家族之星。