Consonance and dissonance
Scientifically a consonant interval is one in which the two notes vibrate at frequencies which have a simple mathematical relationship, e.g. in two notes which are an octave apart the higher note vibrates exactly twice as fast.  If they are a fifth apart the top note vibrates 1 1/2 times as fast as the lower note.  The more complicated mathematically the interval, the more dissonant it will sound.