A hardware security bug is like a secret door in a house that someone can use to get inside without anyone knowing. It's a problem with the way a computer or device is built that can make it easier for a bad person to do bad things with it.
Imagine you have a toy that has a special button that makes it do something cool, like light up or make noise. If someone discovered that if they held down that button for a certain amount of time it would make the toy break and stop working, that would be like a hardware security bug.
In a computer or device, this bug could allow someone to get into it and access information they're not supposed to have, like passwords or personal data. Manufacturers and engineers work hard to find these bugs before someone bad does, and they release updates to fix them.
It's important to make sure you keep your computer and devices up to date and don't click on suspicious links or download things from unknown sources to help keep them safe.