What does it mean to arrange a piece of music? (keep in mind we are talking about writing here)

The following terms are all associated with harmony except: triad: the simplest chord..it is made up of three notes that are usually spaced one note apart-do, mi, and sol in the do-re-mi (major) scale. consonance: a combination of notes that is considered stable and without tension. phrases: where melodies are often made up of shorter sections. chords: the smallest harmonic unit is one consisting of two notes, but we usually have three or more notes played or sung together to create harmony. those groups of notes make up what are called chords. dissonance: a combination of notes that is considered unstable and tense, so much that they sometimes sound as if they are fighting with one another.