When English speakers use the phrase “bread and butter” in a figurative context, they are talking about someone’s livelihood or source of income. This idiomatic way to describe the ways that people make a living is often useful in informal conversations about work and occupation. It’s common for English speakers to refer to something as “my bread and butter,” meaning that it is his or her primary income.
In general, this phrase seems to be an abstraction of the various consumer goods that people buy with their wages. In other words, it’s a person’s job or source of income that allows them to buy bread and butter, as well as jam and other food, and an assorted variety of goods and services. As a popular idiom, English speakers have shortened the grocery list to just “bread and butter,” in the same way that some people refer to a minimal menu as “bread and water,” or in the way that English speakers also might quote the Bible, saying “man does not live by bread alone,” where bread is a generic designation for food. In fact, the simple word “bread” is also often used to mean money.
A similar phrase, “guns and butter” refers in a more general way to items that are desired by a group of people. This is most often used in the political context to talk about the demands of a constituency. The meaning of this idiom is a contrast of defense spending and social spending items, where a government or official authority may have to make tough decisions between “guns” or “butter,” and a constituency that wants both could be said to demand “guns and butter.” For instance, if a state government has to choose between expanding the police force and early education programs, they have a “guns and butter” problem.
Another idiomatic phrase related to bread and butter is used to express a certain life philosophy. If someone says “the bread always falls buttered side down,” they are expressing the idea that if there is an uncertain outcome, that outcome is likely to be bad, or that bad luck is the norm in their life. Here, the bread might be seen as prosperity or financial wealth, and the falling butter side down is a negative outcome that may negatively affect a person’s livelihood or financial future.