It results from the fact that waves reflect ("scatter") from objects which are equal in size to (or smaller than) the wavelength of the wave, but simply "step over" objects which are smaller than the wavelength. This is observed in water waves; why is it that waves aren't stopped by a small rock, but are stopped by an island?

A short-wavelength wave is stopped or reflected

A long-wavelength wave is not affected

It's also true of light waves. Tyndall scattering results from very small particles suspended in air or water. Light of short wavelengths (like blue light) is reflected back, or scattered, while light of longer wavelengths (like red light) passes right through.