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 altered. For example, a key signature with one sharp indicates the key of G major or E minor. Key signatures help musicians quickly identify which notes to play as sharp or flat throughout the piece. This system simplifies reading music by eliminating the need to write accidentals for every occurrence of altered notes. Understanding key signatures is essential for interpreting and performing music accurately.