Rainwater that falls from the sky is considered to be very pure but in actuality the rain water is not as pure as it's thought to be.
In the sky there are impurities and other particles that the rain water can come into contact with when the rain falls from the sky.
So that means that rain water is not the purest when it falls from the sky but rain water is still pretty pure before it reaches the ground and it's safe to drink.
You can leave a glass outside and let it collect rainwater from the sky and then drink it and it will be safe and pretty tasty to drink.
I've done that before and the rainwater was really good and refreshing to drink and it's likely much healthier for you than the water from your city tap that contains chemicals such as chlorine.