bread and butter

1. Means of support; livelihood.
2. The essential sustaining element or elements; the mainstay: “As ever, politics, vulgarity and sentimentality were the bread and butter of the Academy Awards” (David Ansen).

noun

The means needed to support life: alimentation, alimony, bread, keep, livelihood, living, maintenance, subsistence, support, sustenance, upkeep. See money.

1.  The essential, sustaining element, as in The quality of the schools is the bread and butter of town property values. This idiom alludes to a basic food, bread spread with butter. [c. 1700]
2.  Means of livelihood, as in John's job is the family's bread and butter. [First half of 1700s]
3.  Ordinary, routine, as in Don't worry about it; this is just a bread and butter assignment. [Second half of 1800s]

The noun has one meaning:

Meaning #1: the financial means whereby one lives
  Synonyms: support, keep, livelihood, living, sustenance