bread and butter不只有面包和黄油的意思，还有(某人或公司的)主要收入来源；谋生之道。

【例句】
With domestic lending still its bread and butter, much depends on continued economic growth in China.
鉴于国内贷款仍然是工行主要的收入来源，工行将在很大程度上依赖于中国经济的持续增长。

Gardening is my bread and butter right now.
眼下园艺工作是我的谋生之道。