bread and butter
noun /ˌbred ən ˈbʌtə(r)/
  /ˌbred ən ˈbʌtər/
[uncountable]- slices of bread that have been spread with butter
- a piece of bread and butter
 - If you're still hungry, have some bread and butter.
 
 - (informal) a person or company’s main source of income