The established keyword indicates that packets belong to an existing connection if the Transmission Control Protocol (TCP) datagram has the Acknowledgment (ACK) or Reset (RST) bit set
see the below example
interface vlan 20
description Vlan 20 interface
ip address 192.168.20.1 255.255.255.0
ip access-group 100 in
interface vlan 10
description Vlan 10 interface
ip address 192.168.10.1 255.255.255.0
access-list 100 permit tcp 192.168.10.0 0.0.0.255 192.168.20.0 0.0.0.255 established
above command will do
1) permit any tcp comming from vlan 10 netwrok to 192.168.20.0
2) deny any tcp traffic initiating from 192.168.20.0 to vlan 10