A security vulnerability is like a hole in a wall: it's a weakness in something (like internet security, computer software, etc.) that can be taken advantage of to harm or steal from people. For example, hackers might use security vulnerabilities to get into your computer and steal your private information like bank account numbers or passwords. It's important to keep an eye out for security vulnerabilities and make sure that the places where you store your information (like the websites you visit) are safe and secure.