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. The light reflects inside the droplet and separates into different colors. It bends again when it leaves the water.
To see a rainbow, the Sun is usually behind you and the rainbow is on the opposite side. 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.