[Bracketed] words are used when doing direct quotes to change words, or add extra words on a direct quote. This is done because usually the quote might refer to something that is not in the text, so you have to add that context for the quote to have any meaning at all. For example, you can have the following paragraph:
Andrew Ryan built Rapture as an Ayn Randian utopia. By 1960, it had been ravaged by civil war, while he remained locked down inside his own office.
If I want to do a direct quote from this, I would have to do it this way:
"By 1960, [Rapture] had been ravaged by civil war, [...]"
As you can see, I substituted it for Rapture.