Photoelectric Effect
Photoelectric Effect is when light hits a metal and knocks tiny bits of charge loose from its surface.
Think of it this way
A bowl holds marbles on a flat surface. You flick each marble one at a time. A soft flick does nothing. Only a flick hard enough sends a marble off the edge. More hard flicks send more marbles flying, but no number of soft flicks will move even one.
Explaining photoelectric effect by grade level
Think about a smoke alarm in your home. Inside, light shines on a small piece of metal. That light pushes tiny bits off the metal. When smoke blocks the light, the alarm knows something is wrong.