An intrusion-detection system is like a superhero that is always watching to make sure no bad guys sneak into a house or a computer.
When you have an intrusion-detection system, it can tell you if someone is trying to get into your computer or your house without your permission.
The superhero has special sensors that can detect if someone is trying to break in through the windows or doors, and it will make a warning sound to let you know that something is wrong.
Similarly, the intrusion-detection system keeps an eye on the data that comes into your computer and it can tell you if any bad programs or viruses are trying to get in.
So, it's like a watchful protector that keeps your computer and your house safe by detecting the intruders and alerting you.