When writers insert or alter words in a direct quotation, square brackets—[ ]—are placed around the change. The brackets, always used in pairs, enclose words intended to clarify meaning, provide a brief explanation, or to help integrate the quote into the writer’s sentence.
https://writingcommons.org/open-text/research-methods-methodologies/integrate-evidence/incorporate-evidence/453-inserting-or-altering-words-in-a-direct-quotation