What does brckets around accidentals in music mean?

Answer 1

Well, accidentals are notes that aren't on the scale of a key. Some pieces are written in different keys and this is difference is indicated at the beginning of the piece with a sharp, or flat sign next to the treble cleft (depending on what instrument). So an accidental is the sign (flat or sharp) that changes the key of a note somewhere in the measure.

Brackets are just used to clarify the 'status of the note.' If a note, like C for example, had been neutralized in some previous measure, and then there is another C in the next measure, there might be an accidental enclosed in brackets, saying that this C should not be neutralized. It's just cautionary.