When you select HTTPS as your listener, you will be asked to select a certificate using one of four different options available.
The first two options relate to ACM.
An ACM is the AWS Certificate Manager and this service allows you to create and provision SSL/TLS server certificates to be used within your AWS environment across different services.
This integration with ACM simplifies the configuration process of implementing a new certificate for your elastic load balancer and as a result, it's the preferred option.
The last two options allow you to use a third-party certificate by using IAM as your certificate manager and you would select this option when deploying your ELBs in regions that are not supported by ACM.