Normally it is supposed to be in brackets [], but the brackets and parentheses are often used interchangeably.
It basically means: "the quote is like this, but I added this word in myself."
Like if I say: "I hate my university. It has the most ridiculous requirements." and someone from a big business magazine wanted to quote me in 20 years when I'm king of the world, but he didn't want to quote my first sentence, he'd probably say: "[My university] has the most ridiculous requirements."
Meaning "he is talking about his university, but he didn't use those exact words."
-Warik