The functionality of a web application firewall (WAF) is to filter, monitor, and protect web applications from various internet threats, such as SQL injection, cross-site scripting, and other vulnerabilities. WAFs inspect incoming traffic and apply predefined rules to identify and block harmful requests before they reach the application's backend. By providing this layer of security, WAFs help safeguard sensitive data, maintain application availability, and ensure compliance with regulatory standards, making them essential components of web security strategies.