This is in a part a physics lesson part music lesson.
When a string vibrates it actually creates many different pitches…the overtone scale.
It is these partials that create the color and richness of the tone.
A sine wave only has one pitch-the fundamental.
Synths use multiple wave form generators to recreate the overtone scale or, create a different series of notes ABOVE the fundamental to give it character.
Why is this important to consider as a composer/orchestrator etc?
When you are dealing with a live group of instruments all of the overtones clash together because they are in an acoustic, natural setting. So, if you are not conscious of what the overtones are doing you are basically not in control of the sound you are attempting to create.
Listen to anything by Mozart….he’s the best that ever lived in this regard. He was able to get power, subtlety etc. out of an orchestra that was smaller in size than what we are accustomed to today.
It’s not about the number of bodies you have in the orchestra, it’s really all about understanding how the instruments available work together.