Other forms: square brackets
Square brackets are punctuation marks that are used in pairs to enclose parts of a text. In a direct quote, square brackets show that a writer added the enclosed text for clarification.
If you see square brackets in a quotation, you know the person being quoted didn't literally say what's inside them. A quote like "I handed the wallet to Tommy" might be changed by a journalist to "I handed the wallet to [my brother]" in order to make the meaning clearer to the reader. Those square brackets clearly communicate that the speaker meant, but didn't say, "my brother." Square brackets are also used in legal and academic writing, math, and programming.