bread是面包，butter 是黄油，面包和黄油是老外最基本的生活需求，所以 bread and butter 就是指“生计，主要收入来源”的意思。

In my book, I assumed that writing was her bread and butter, but she told me that it's kind of a hobby.

我以为她以写东西为生，但她却告诉你这只是她的一个爱好。

- I've got a new job. It's my bread and butter from now on.

- 我找到了个新工作。从现在开始。它就是我的饭碗了。

- Good for you!

- 恭喜啊！

下面我们再来看几个和这个口语表达有关的例句：

- How does he earn his bread and butter?

- 他怎样谋生的呢？

- Beats me.

- 我不知道。

I can't play hooky from work. That's my bread and butter.

我不能旷工，那可是我的饭碗。

Our customers are our bread and butter, so treat them with respect.

我们的客户是我们的收入来源，所以要尊重他们。

另外，bread and butter中间加上连字符还可以作形容词，意思是“生计的，基本的”。

She worries about everything, even the bread-and-butter tasks of everyday life.

她什么都担心，甚至是日常生活的基本任务。

出处：头条号 @VOA英文口语