Pseudo-parameters are predefined parameters provided by AWS CLoudFormation. You can use them within a Ref or a Sub function to dynamically populate values. Pseudo-parameters available to use within a CloudFormation template include AWS::AccountId, AWS::NotificationARNs, AWS::NoValue, AWS::Partition, AWS::Region, AWS::StackId, AWS::StackName, and AWS::URLSuffix.

Read more about pseudo-parameters at https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/pseudo-parameter-reference.html.