The parameters you described above are used for the following:

• NeptuneBulkloadIAMRoleArn - This is an IAM role setup to run the loader command. Instructions for setting this up found here.
• NeptuneClusterEndpoint - This is the endpoint of your Neptune database, it will be accessible either from the console or the CLI.
• NeptuneLambdaIAMRoleArn - This allows you to pass in your own role the Lambda should use, if not specified the CloudFormation stack should make one for you.