Who decides who gets in where in your computer - and why a hole in the heart of this system is the most dangerous thing that can happen.
Every program on your computer wants something: to show something on the screen, save a file, use memory. The operating system decides who gets in where and when - it's the doorman of the building. Programs knock on the door, and it decides: this room - yes, that one - no.
Windows, macOS, Android are the doormen you know - you see them on the desktop, on the laptop, on the phone. Linux is a different doorman: free, built by thousands of people around the world who improve it together. You don't sit in front of it - it stands behind the scenes, on the servers that run sites, banks, hospitals. Almost the entire internet passes through its hands, even though you never see it.
Ubuntu is the most widespread uniform of this doorman. Linux has many versions - different people have dressed it differently, for different needs. Ubuntu is the one most companies trust, because it's tested, maintained and renewed regularly. Like the doorman most buildings in the neighborhood pick, because they know he does his job right.
The kernel is the doorman's heart - the part of it that makes the real-time decisions: who gets in, who doesn't, who sees what. It's not the whole building, it's the doorman himself, behind the desk. If someone finds a hole in the kernel, they don't steal one key - they sit down in the doorman's seat. From there they see everything and let in whoever they want.
The doorman guards everything
I look at the operating system as the first building manager I give access to, before I've even put up a camera or a lock. If he isn't secure, nothing else on top matters - the guards, the passwords, all of it is decoration on a shaky foundation. So when I build a system, the first question is who the doorman is, and whether I can trust him.
I've picked Ubuntu many times for that exact reason: thousands of eyes check it constantly. The peace of mind comes from knowing who's guarding the door.