Macaroon.serialize

You're seeing just the function serialize, go back to Macaroon module for more information.
Link to this function

serialize(macaroon, atom)

Specs

serialize(Macaroon.Types.Macaroon.t(), :binary | :json) ::
  nil
  | {:error,
     %{
       :__exception__ => any(),
       :__struct__ => Jason.EncodeError | Protocol.UndefinedError,
       optional(atom()) => any()
     }}
  | {:ok, binary()}

Serializes a Macaroon into a more transmittable format

2nd argument for "type" can be :binary or :json