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 consistently throughout a piece. They simplify 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.