A musical key is a system that defines the scale and tonal center of a piece of music. It indicates which notes are most important and how they relate to each other. For example, a piece in the key of C major primarily uses the notes from the C major scale, which includes C, D, E, F, G, A, and B. Each key has a unique sound and mood, influencing the overall feel of the music. Keys can be major, which often sound happy, or minor, which tend to sound sad or somber. Understanding musical keys helps musicians and composers create and interpret music effectively.