An AMI is an encrypted Amazon Machine Image. When you launch an EC2 instance, you're launching an instance of a previously created AMI. The idea here is that you launch some "base" instance to get started, install necessary software, configure, and otherwise get this instance into a shape that you'd want to deploy repeatedly. To do that, you'll take the instance you previously created and create an image out of it. After you have an image, you can then launch multiple instances of it, thereby achieving the provisioning that brought you to AWS EC2 in the first place.
Once you have confirmed that the instance you created previously is working, you'll select that image in the AWS Console and then select "Create Image". It'll take you through some screens. Name it clearly. Tag it appropriately. Then let it do its thing.
This will take some time, and it'll shut down the instance from which you are generating the AMI
You will have the option of making this AMI public or private. As of this writing, I believe you need to keep it private due to distribution restrictions in the ColdFusion EULA