I'm confused as to what the purpose of AssumeRolePolicyDocument is.
It seems like to create a role all you need is a parent name that you can attach policies to. It seems like AssumeRolePolicyDocument is that parent name but somehow there's more to it. What is this doing?

 "AssumeRolePolicyDocument": { "Version": "2012-10-17", "Statement": [ { "Effect": "Allow", "Principal": { "Service": [ "lambda.amazonaws.com" ] }, "Action": [ "sts:AssumeRole" ] } ] },