Mappings are hash tables, they can be referenced to perform searches by key name and retrieve specific values. We want our ec2-instance.yaml template to be portable across geographic regions but also to stay in compliance for the authorized corporate AMIs.

The pseudo function AWS::Region is used to replace the current region code into the function FindInMap call, this is specially useful to retrieve values, for example, account number.

The Fn::FindInMap function receives three parameters, the MapName, the TopLevelKey, and the SecondLevelKey. In this case, the function will be replaced with this values:

ImageId: !FindInMap [RegionMap, us-east-1, HVM64] 

And passed to the resource as:

ImageId: ami-0ff8a91507f77f867 

A completely refactored example is provided in the following link: https://github.com/gabanox/Certified-Solution-Architect-Associate-Guide/blob/master/chapter20/ec2-instance-full.yaml. Take your time to analyze it fully.