Hello, the action you're using can be used to retrieve an existing object from an AWS S3 bucket. These parameters are required to get the object/file you're looking for.
Are you already using the bucket? If so, you should be able to see which bucket/key combination you need here. Potentially, it's helpful to read something about the concepts of Buckets and Objects in S3.
https://docs.aws.amazon.com/AmazonS3/latest/userguide/Welcome.html#BasicsBucket
Hopefully, this helps.