A content delivery network (CDN) is a system of distributed servers that delivers web content to users based on their geographic location. By caching content closer to the user, CDNs reduce latency, improve loading times, and enhance the overall user experience. They are crucial for handling large traffic volumes and ensuring reliability, especially during peak usage periods. CDNs also provide security features, such as DDoS protection and secure token authentication, making them essential for modern web applications and services.