The letters "XSS" stand for Cross-Site Scripting. It is a security vulnerability typically found in web applications that allow attackers to inject malicious scripts into content delivered to users. When a victim interacts with the compromised web application, these scripts can execute in their browser, potentially stealing sensitive information, such as cookies or session tokens, or performing actions on behalf of the victim. Protecting against XSS involves validating and sanitizing user inputs, implementing content security policies, and employing secure coding practices.