bread and butter

/B R EH1 D AE1 N D B AH1 T ER0/
noun
  1. 1

    That which is central or fundamental, as to one's business, survival, or income; a staple or cornerstone.

    โ€œThey will do some machining if you ask them, but sheet metal has always been their bread and butter.โ€

interjection
  1. 1

    Said when two people walking together are temporarily separated by an obstacle in order to indicate that they belong together.

Source: https://en.wiktionary.org/wiki/bread%20and%20butter