What is the purpose of brackets in a quotation?

When quote wouldn't make sense by itself words are added that aren't from the original text. These words are put in brackets.

Brackets are placed around words that are altered in a quotation ... for example, someone might say ''She was born from the blossom'' but someone quoting the person may change it too ''[Thumbelina] was born from the blossom'' to specify what they're talking about, so u place brackets around the part that was changed.