CloudFormation - Join example in YAML

ec2-join-tag-example-yaml.yml

Parameters:
  App:
    Description: "Examples: blog, api"
    Type: String
  Env:
    Description: "Examples: prod, stag, dev"
    Type: String
  Role:
    Description: "Examples: web, worker, scheduler"
    Type: String
Resources:
  EC2Instance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: t2.micro
      KeyName: 'default' # keypair must already exist
      ImageId: ami-6869aa05 # AMI us-east-1
      Tags:
        -
          Key: Name
          Value: !Join [ '-', [!Ref App, !Ref Role, !Ref Env] ]

Thank you for such a simple explanation!