Simple Definition:

The Tyndall Effect is the scattering [spreading out in all directions] of light by particles in a substance [material]. When a beam of light passes through a substance [material] with small particles, the light gets dispersed [spread out] in different directions. This causes the light to become visible as a beam or a cone of light [ray of light]. Examples of substances [materials] that exhibit the Tyndall Effect include fog, smoke, and milk.

Very Simple Definition:

The Tyndall Effect is when light makes tiny things visible. For example, when you shine a flashlight through fog or smoke, you can see the light beam [light line] because it scatters [spreads] off the tiny particles in the air.