UV Radiation
UV radiation is light from the sun you cannot see, but it can burn your skin.
Think of it this way
A sunlamp sits on a table and shines light down onto a tray of bread. Some light you can see makes the bread warm and toasty. The lamp also sends out rays you cannot see that travel in straight lines like the visible light. These hidden rays hit the bread and break down its outer layer, the same way sunlight burns your skin.
Explaining uv radiation by grade level
The sun sends out light you can see and light you cannot. Some of that hidden light can hurt your skin. Sunscreen acts like a shield. It soaks up the hidden light before it reaches you.