A web application firewall (WAF) is a security system that monitors and filters incoming traffic to a web application, in order to protect against malicious attacks that may exploit vulnerabilities in the application or its servers. WAFs are typically deployed as a network appliance or as a module in a web server, and they use a combination of technologies such as firewalls, intrusion detection and prevention systems, and signature-based detection to filter incoming traffic and block known malicious requests. 

WAFs can be configured to enforce various security policies, such as blocking requests that contain potentially malicious payloads, blocking requests from known malicious IP addresses, or blocking requests that violate specific application-level rules (e.g., preventing cross-site scripting attacks). WAFs are an important part of a comprehensive security strategy for web applications, as they can help to protect against a wide range of attacks, including cross-site scripting, SQL injection, and other types of injection attacks.