1. a basic unit from which sth is built
the building blocks of life/success/society
'Building blocks' are blocks which can be used to create a building. Figuratively 'building blocks' are the foundations upon which other things are built. "A customer-facing role is a good building block for a management position later on in life." "Ballet is considered the building block for many other types of dance." This expression is used in professional and social contexts.