The Tyndall effect the scattering of a beam of light in a colloidal dispersion. It is a way of telling if a liquid is a true solution or not.
In a true solution a beam of light will pass through without being scattered. The molecules in a solution are evenly mixed. This causes the light to pass smoothly through without being scattered.
In a colloidal dispersion the particles are suspended in the liquid.
It may not be possible to see the suspended particles with the naked eye or even with a microscope as they are so small. However the photons in a beam of light are even smaller than the particles and will be scattered by the particles.
So the Tyndall Effect says that if a beam of light is scattered the liquid is a colloidal dispersion.