What is an Access Control List (ACL)?
D2 · Threats · CompTIA Security+ SY0-701An ACL (Access Control List) is an ordered set of rules that a router, firewall, or operating system uses to permit or deny traffic and access to resources. Each rule (called an ACE — Access Control Entry) specifies a source, destination, port, and action (permit/deny).
On the exam, ACLs are a network-layer control. Firewall rules are often implemented as ACLs. "Implicit deny" means any traffic not explicitly permitted is blocked.