Mate.Driver.start

You're seeing just the callback start, go back to Mate.Driver module for more information.

Specs

start(session :: Mate.Session.t(), host :: String.t()) ::
  {:ok, Mate.Session.t()} | {:error, String.t()}

Starts the driver, for example create an SSH connection, start a docker container, or whatever else might be needed for your new session.