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 receive fast and reliable access to applications by balancing traffic, improving security, and enhancing performance. ADCs can also provide features like load balancing, SSL offloading, and application firewall capabilities. By distributing incoming traffic across multiple servers, ADCs prevent any single server from becoming overwhelmed, which can lead to slowdowns or outages. They are commonly used in data centers and cloud environments to ensure that applications run smoothly and efficiently, ultimately improving the user experience.