A security group acts as a virtual firewall for your EC2 instances to control incoming and outgoing traffic. Unlike network ACLs, security groups are stateful; if you send a request from your instance, the response traffic for that request is allowed to flow in regardless of inbound security group rules.

Traffic Regulation

1
Inbound Traffic: Filters traffic coming into the instance based on protocol, port range, and source IP/Security Group.
2
Outbound Traffic: Filters traffic leaving the instance to specific destinations.
3
Default Deny: By default, all inbound traffic is blocked until you add allow rules.
4
Stateful Filtering: Automatically allows return traffic for established connections.