Composer is Dependency Manager for PHP.
This is de-facto standard packages manager and CleverStyle Framework have great support for it.
Usual thing is to create composer.json file in the root of the project and declare project dependencies there.
System is self-contained, so it comes with all necessary dependencies bundled and thus doesn't have composer.json in the root directory.
However, if you'll create it and install some packages - vendor directory will be detected by system and all your dependencies will be automatically available anywhere you need them.
composer.json is great, but CleverStyle Framework has own packaging system which is designed specifically for this purpose; sometimes such packages may rely on external dependencies, but having them bundled is not a very good idea because other components might need them as well and it will be very difficult to control conflicts.
For this purpose there is Composer module. It allows to declare in meta.json not only dependencies on CleverStyle Framework-specific, but also on regular Composer packages.
As the result, when you install such package - all Composer dependencies will be resolved and installed automatically (if there are no conflicts). Moreover, you do not need any access to terminal or even have Composer installed on server - Composer module will do everything for you.
For specific details refer to module's readme.