Midi files are provided of major and minor triads. Along with the thirds and perfect fifth previously mentioned, these are 5-limit consonances . A chord is a 5-limit cosonance if it contains no other intervals than these, or octave equivalences. In traditional harmony, only the 5-limit is recognised, so such chords are simply defined as consonances. Other chords are dissonances.
Consonances have a smoother, or more restful sound than dissonances.
At least, that's partly true. The diatonic scale has 6 5-limit triads in it. You could say that the diatonic scales is defined by having 6 5-limit triads, but that's dodgy historically. A complex note is made up of overtones or partials . These give an instrument its timbre or tone colour.