In Magento2 module is one of the components which provides a certain feature / functionality. For an example, module-checkout provides checkout functionality in the store front. Similarly each module has it's own features and and they can also be dependent on other modules.  I insist you to go through [Magento 2 official docs for proper knowledge][1] 


  [1]: https://devdocs.magento.com/guides/v2.3/extension-dev-guide/bk-extension-dev-guide.html