Each colour stands for one of the five great elements that make up the world: blue for the sky, white for the wind and air, red for fire, green for water, and yellow for the earth. Together they show how everything in nature is connected.
The flags are printed with prayers, symbols and pictures. People believe that when the wind blows through the flags, it carries those good wishes out into the world, like seeds being scattered.
Prayer flags are never taken down by being thrown away. As the months pass, the wind, sun and rain slowly fade them. When they are completely faded, new flags are hung up next to the old ones - the idea is that the old wishes are 'finished' and have already drifted away.
You'll see them at mountain passes, bridges and on the roofs of houses. In a strong Himalayan wind, the whole hillside can flap and rustle with thousands of bright colours at once - one of the most beautiful sights in Nepal.