An Access Control List (ACL) is a set of rules that determines who can access specific resources in a computer system or network. It specifies which users or groups have permission to perform actions like reading, writing, or executing files. ACLs help maintain security by ensuring that only authorized individuals can access sensitive information. ACLs are commonly used in various systems, including operating systems, databases, and network devices. By defining permissions for each user or group, ACLs help protect resources from unauthorized access and potential threats, making them an essential component of cybersecurity practices.