An Application Delivery Controller (ADC) is a network device that helps manage and optimize the delivery of applications over the internet. It ensures that users can access applications quickly and reliably by balancing the load among multiple servers, which prevents any single server from becoming overwhelmed. This improves performance and enhances the user experience. Additionally, an ADC provides security features, such as protecting against cyber threats and ensuring that data is transmitted securely. By using technologies like SSL offloading and web application firewalls, an ADC helps organizations maintain high availability and security for their applications, making it a crucial component in modern IT infrastructure.