What is the purpose of brackets in a quotation?

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.

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