What role do security groups play in managing access to Amazon EC$2$ instances?

Security groups identify the public and private keys required to connect to an Amazon EC$2$ instance.

Security groups control inbound access to the subnet that an Amazon EC$2$ is associated with.

Security groups define the AWS Identity and Access Management $($IAM$)$ roles that can access an instance.

Security groups provide a set of rules to control traffic to or from an instance.