Which AWS service or feature acts as a firewall for Amazon EC2 instances?

The AWS service that acts as a firewall for Amazon EC2 instances is called Amazon VPC (Virtual Private Cloud). Within Amazon VPC, there is a feature called Security Groups that acts as a virtual firewall for your EC2 instances to control inbound and outbound traffic.

Here are the steps to use it:

1. Sign in to the AWS Management Console and open the Amazon VPC console at https://console.aws.amazon.com/vpc/.

2. In the navigation pane, choose 'Security Groups'.

3. Choose 'Create security group'.

4. Type a name for the security group, and a description.

5. In the VPC list, choose the ID of your VPC, and then choose 'Yes, Create'.

6. Select the newly created security group from the list, choose the 'Inbound' tab, and then choose 'Edit'.

7. In the dialog box, choose 'Add Rule' for each type of inbound traffic that you want to allow.

8. For 'Type', choose the type of traffic, for 'Source', specify the source, and then choose 'Save'.

You can also add outbound rules by selecting the 'Outbound' tab and following the same steps.

Remember, by default, a security group includes an outbound rule that allows all outbound traffic. You can remove the rule and add outbound rules that allow specific outbound traffic only. If your security group has no outbound rules, no outbound traffic originating from your instances is allowed.