What is the significance of the sub-tonic in music theory and how does it contribute to the overall harmonic structure of a piece?

The sub-tonic in Music Theory is the seventh note of a scale, one step below the tonic. It is significant because it creates tension and a sense of resolution when it resolves to the tonic. This contributes to the overall harmonic structure of a piece by adding depth and complexity to the music, creating a sense of movement and resolution within the harmony.