'XSS' stands for Cross-Site Scripting in web security. It is a type of security vulnerability that allows an attacker to inject malicious scripts into trusted websites. When a user visits an affected page, the injected script can execute in their browser, potentially stealing sensitive information like cookies, session tokens, or personal data. XSS attacks can compromise user security, leading to data breaches or unauthorized actions. Mitigating XSS attacks involves implementing proper input validation, utilizing Content Security Policy (CSP), and encoding outputs to protect web applications from malicious scripts.