Imagine you have a hard boiled egg in your hand. If you tap it gently with your finger, it might crack a little bit. But if you tap it too hard, it could break completely and make a big mess!
Now imagine your head is like an eggshell. Some people have very thick and strong eggshells, which means they can bump their heads pretty hard without getting hurt too badly. But some people have very thin and fragile eggshells, like a real eggshell that would break easily.
If someone with a thin and fragile eggshell (head) gets bumped or hit, their eggshell (head) can break or crack much more easily than someone with a strong eggshell (head). This is what doctors call the "eggshell skull" phenomenon.
Basically, it means that someone who already has a weak or fragile skull is more likely to suffer severe injuries if they hit their head, even if the bump or blow isn't very hard.
So it's important to protect your head as much as possible, especially if you know you have a thin or fragile eggshell (skull) that could easily break under pressure. You can wear a helmet or avoid risky activities that could result in head injuries to keep yourself safe.