great-grandparents
Refers to the parents of one's grandparents. A straightforward kinship term.
My great-grandparents were born in the Meiji era.
祖父母 means grandparents, while 曽祖父母 means great-grandparents (one generation further back).
曾祖父母 is a variant kanji spelling using 曾 instead of 曽; both are read そうそふぼ and have the same meaning.
Compound of 曽 (great-) + 祖父母 (grandparents). The prefix 曽 indicates one generation further back.