Brackets are used to indicated text that has been added or modified in a quote, usually to make it fit the grammar in a sentence.
In this case, in the original quote "it" was not the first word of the sentence and so would not be capitalized. But they left something off at the beginning, and so they had to capitalize the "i". To show that this is not quite an exact quote, they put brackets around it.
You may see a name or other identification in brackets if the part of the original statement that identified the person or thing is not included in the quote. Like suppose someone said, "Al is an honest man. Bob doesn't trust him, but he wouldn't lie." You want to quote this, but for the sake of brevity you want to leave off the first sentence. Without that, the reader has no idea who "him" and "he" are. So you can write, "Bob doesn't trust [Al], but [Al] wouldn't lie."
Finally, you may see a whole word in brackets when the writer had to change the tense of a verb or some such to fit the sentence. Like someone says, "I frequently post on Stack Exchange." You want to quote this, but you want to shift the "I" to the name of the person who made the statement. You could write, "Bob said that he 'frequently post[s] on Stack Exchange'."
Of course you should only do this if it doesn't change the meaning of the quote. Quoting "I will do X" as "I will [not] do X" is not acceptable! Subtler changes could be misleading. That's why we use the brackets: to warn the reader that you've made a subtle change that you think does not alter the meaning, but that the reader should be on guard about.