A triad is made up of three notes (root, third and fifth) played simultaneously. The music we listen to today is made up of primary and secondary triads. These are related to a keynote from which a scale is made.
The following is a guide to how the degrees fit into music. There may be some exceptions to the following.
Tonic This is the main note of a key and normally the note that a piece of music finishes on. Supertonic This is the second note in a scale. Mediant This is the third note of a scale and also the second (middle) note of the tonic triad. This is a powerful note as it defines the tonality of a key. Subdominant This is the fourth note of a scale and also the first note of the subdominant triad. Other degrees that make up this triad are the submediant and the tonic. Dominant This is the fifth note of a scale. It is the third note of the tonic triad and also the first note of the dominant triad. The name dominant is given because this note is in two primary triads. It sounds very strong like the tonic. Submediant This is the sixth note of a scale and also the second (middle) note of the Subdominant triad. Leading note This is the seventh note of a scale leading on to the tonic. It is also the second note of the Dominant triad. This note sounds very strong and is powerful because it tends to lead your ear towards the tonic.