Bread and butter

The main work or activity that gives someone or a business most of its income.

Examples

Wedding photos are the bread and butter of his business.
Translation is my bread and butter, but I write novels in my free time.

How to use it

Neutral. As an adjective it is hyphenated: bread-and-butter issues (basic, everyday ones).

cash cowA product or business that brings in a steady, reliable profit.