Magnetic field that protects Earth from cosmic radiations.
Earth has a visible atmosphere that protects it from meteorites and small debris bombardment. But Earth also has an invisible magnetic field that protects it from cosmic radiations called the Magnetosphere.
Planets having active magnetospheres, like the Earth, are capable of mitigating or blocking the effects of solar radiation or cosmic radiation, that also protects all living organisms from potentially detrimental and dangerous consequences.
Wikipedia, Magnetosphere, 2018.
Without this magnatic field life would be impossible on Earth's surface.
[Quran 21.32] And We made the sky a protected shield and they turn away from its sign.
The visible atmosphere sheilds Earth from meteorites and small debris bombardment. But to understand why this verse said a "protected shield" you have to look at Mars. Mars had an atmosphere however it lost it because it didn't have a magnetic field. However Earth's atmosphere is protected by this invisible magnetic field. This is why this verse said "protected shield" because this atmosphere (our visible sheild) is also being protected.
How could an illiterate man who lived 1400 years ago have known that Earth has a protected shield?