When a speaker cone is moving in and out at 100hz you will hear a sine bass sound. But when the cone is also vibrating at 10000hz while moving in and out at 100hz there will be another pitch on top of the bass sound.

So different wavelengths added together will tell your brain there are different pitches at the same time.

When all those different wavelengths are using the same volume at the same time you can imagine that all those added waves will become a mess. That is what is called noise.

But when a high pitch wave is 'traveling' on top of a low pitch wave your brain can distinguish the different pitches.