What is ACL: Access control list or ACLs are a set of if-then rules set on a router to allow or deny a specific group of IP to send or receive traffic from your network into another network.
When you apply an ACL to a route interface  for incoming traffic then every incoming packets will be compared with your ACL first, if a match is found then either permit or deny action will take place according to your configuration of the ACL. If there are more than one ACL and if there is no match after comparing the incoming packets with the first ACL, then the second ACL will be matched and so on. Remember there is an implicit deny after an ACL. That is why after permit or denies any IP or IP range you must add the command: access-list ip permit any any.
For example in your network you want that no computer or devices from 172.16.0.0 network can send traffic to your network. To implement this rule you need to write and ACL that will tell your router to discard all the traffic from 172.16.0.0. Now, let see how to implement this ACL into a router.
Rourter#configure terminal
Router(config)# access-list 10 deny ip 172.16.0.0 0.0.255.255
Router (config) #access-list 10 permits ip any any
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