A key signature is a set of sharp or flat symbols placed at the beginning of a musical staff. It indicates the key of a piece of music, which determines the scale used and the specific notes that are sharp or flat throughout the composition. Key signatures help musicians understand which notes to play consistently, making it easier to read and perform the music. There are 15 different key signatures, each corresponding to a major or minor scale. For example, the key signature of C major has no sharps or flats, while G major has one sharp. Understanding key signatures is essential for musicians to interpret and play music accurately.