Access Control Lists, also known as ACLs, are used to allow or block traffic. ACLs can also be used to identify what traffic needs to have NAT performed on or what traffic needs to be prioritized based on QoS policies.
ACLs are commonly used on switches and routers to control the traffic on its interfaces.
ACLs can block or allow based on source IP, destination IP, TCP port number, UDP port numbers, ICMP, etc.