Ok, you have installed a Yii 2 basic or advanced app for the first time and completed the post install steps. You have picked up a few extensions to install for the first time as well. You would already see a composer.lock file in your application root folder.
This wiki shall be considered deprecated since the Yii installer for composer has been discontinued.
When creating web-application projects, you can add a great variety of different extensions, modules, plugins or scripts to your code-base. But maintaining a stable combination and updating certain packages can become a hard task.