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.
MagnetospherePlanets 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.
Without this magnetic field life would be impossible on Earth's surface. This was known recently, however this was portrayed in the Quran 1400 years before it was discovered.
The visible atmosphere shields 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 shield) is also being protected. (Mars shield was not protected)
How could an illiterate man who lived 1400 years ago have known that Earth has a protected shield?
The Quran (Koran, the book of Islam) contains scientific knowledge that could not have been known 1400 years ago. It ranges from basic arithmetics to the most advanced topics in astrophysics. You are invited to go through those miracles and judge for yourself.