Hi there,
The ASA is a stateful firewall. Providing a connection is permitted the ASA will monitor the state of the connection and permit return traffic.
If traffic from a higher security level is egressing via a lower security level interface, the traffic will be implicitly permitted, providing an ingress ACL hasn't been configured.
If traffic is arriving on a lower security level interface it will need to be explicitly permitted via an ACL.
cheers,
Seb.
