Simplify the following text for a Grade 11 native English-speaking person.
Original: The harmonics have the property that they are all periodic at the fundamental frequency , therefore the sum of harmonics is also periodic at that frequency .
Simplified: Harmonics is a short term for harmonics of a wave motion , or wave motion harmonics . Harmonics are waves added to a fundamental wave ( of any radiant energy , for example sound or electricity ) , having a frequency that is a whole multiple ( an integer ) of the fundamental frequency .