After rain, you may see a colored arc in the sky. A rainbow needs sunlight, water droplets in the air, and a viewer in the right position.
When sunlight enters a droplet, it slows down and bends. Different colors bend by different amounts, so the sunlight spreads into many colors. The light then reflects inside the droplet and bends again when it leaves the water.
The Sun is usually behind you, and the rainbow appears in front of you on the opposite side of the sky. A rainbow is actually a full circle, but the horizon hides part of it from people on the ground. In the right conditions, someone in an airplane may see the complete circle.