The Drupal community has created tens of thousands of contributed modules. Every once in a while, a widely-used and well-written Contrib module is added to Core. This is one of the ways that Drupal Core gets new features. In fact, with the release of Drupal 8, several modules and functions that used to be Contrib are now included in Core. This means less installation, less code for you to update, and a more stable website.
The page at https://www.drupal.org/documentation/install/modules- themes/modules-8 has good explanations of the primary ways to install a module. If you’re working with a developer, ask her the best method as it can vary based on your server configuration, security, access levels, and what’s already installed. Below are the most popular methods.