A rainbow is an arc of color in the sky that you can see when the sun shines through falling rain. The pattern of colors starts with red on the outside and changes through orange, yellow, green, blue, indigo to violet on the inside.
A rainbow is actually round. On the ground, the bottom part is hidden, but in the sky, like from the view of a flying airplane, it can be seen as a round shape.
Rainbows are popular symbols, they usually mean peace and harmony. In the Bible, it says God made a rainbow, after he flooded out evil from the world, a promise to never harm the Earth like that again.