Refers to a stone used as the base or cornerstone of a building, both literally and figuratively.
The foundation stones of this building have been used since the Edo period.
教育は社会の土台石だ。
Education is the cornerstone of society.
土台 is the broader term for foundation or base, while 土台石 specifically refers to the stone used in that foundation.
Compound of 土台 (foundation) and 石 (stone). The exact historical derivation is uncertain, but the word transparently means 'foundation stone'.