An Access Control List (ACL) is a set of rules that tells a router which packets to allow or deny. Think of it as a bouncer at a club checking IDs. Each rule examines packet headers and makes a permit or deny decision.
You configure ACLs on router interfaces to filter traffic entering or leaving your network. Without ACLs, all traffic passes through freely. With ACLs, you control exactly what gets through based on source addresses, destination addresses, protocols, and port numbers.