Extended Access Control List (ACL) - established Keyword
The "established" keyword is used to indicate an established connection for TCP protocol. An established connection can be considered as the TCP protocol traffic originating inside your network, not from an external network.
This means that the packets belong to an existing connection if the Transmission Control Protocol (TCP) segment has the Acknowledgment (ACK) or Reset (RST) bit set.
Router03(config)#access-list 102 permit tcp any any eq www established