Though relative to the listener and impossible to define as fact, there are terms we use to describe what we hear as either good or bad. Consonance (or Repose), is used to describe notes that sound good together. Dissonance (or Resonance), is used to describe notes that do not sound good together, or do not sound in harmony.
Chords are the best way to explore dissonance and repose as it is easy to hear the changing sounds of different chords and associate them by name. Their names (maj3, dim5, min2) help to mentally describe what a chord will sound like without actually playing them. However, since there are so many possible chords and scales, it is necessary to find a better way to describe the sound a combination of notes will make other than melodically and through the use of names.