Another way to define "bread and butter" would be a specific and reliable income producer within a larger set of revenue sources. A CPA (certified public accountant) may handle a lot of different accounts for various clients, but filing tax returns would be his "bread and butter". Other clients may come and go, but the number of tax filers usually remains constant from year to year.
A general practitioner may treat numerous illnesses, but upper respiratory infections could be considered his or her "bread and butter". Those office visits can be counted on to generate enough income to pay the bills every month.