Config.config_env

You're seeing just the macro config_env, go back to Config module for more information.
Link to this macro

config_env()

View Source (macro) (since 1.11.0)

Returns the environment this configuration file is executed on.

In Mix projects this function returns the environment this configuration file is executed on. In releases, the environment when mix release ran.

This is most often used to execute conditional code:

if config_env() == :prod do
  config :my_app, :debug, false
end