Imagine you have a flashlight in the dark and it’s foggy. The light from the flashlight gets scattered by the fog. You see the light shining everywhere in the fog, not just at the target point of the flashlight.

It’s the same with the sun and the air in the sky. Our air is much clearer than fog, but many tiny particles (like dust or water droplets) scatter the light – especially blue light. That's why our sky looks blue!