A little more detailed answer:
The config Files are where you store all configurations. You shouldn't add any username, passwords or other secret informations in them, because they will be in your source control.
All secret informations and all environment dependant informations should be stored in your .env file. With this you can have different configuration values in local/testing/production with just a different .env file.
In your config files you access the information in you .env files, if necessary.
When use what from another answer
example config app/myconfig.php
return [
'myvalue' => env('MY_VALUE', 'bar'), // 'bar' is default if MY_VALUE is missing in .env
];
Access in your code:
config('myconfig.myvalue') // will result in 'foo'