An Access Control List (ACL) is a set of rules that determines who can access specific resources in a computer system or network. Each entry in an ACL specifies a subject (like a user or group) and the permissions they have, such as read, write, or execute. ACLs help protect sensitive data by ensuring that only authorized individuals can perform certain actions. ACLs are commonly used in various systems, including firewalls, operating systems, and database management systems. By implementing ACLs, organizations can enhance their security posture and manage user permissions effectively, reducing the risk of unauthorized access to critical information.