WordPress is a flexible and feature-rich Content Management System (CMS). However, its core software is relatively limited. This is by design; after all, there is no point bloating WordPress core with features that not all site owners will need. Nonetheless, if you build out anything more than a super-basic WordPress website, you will soon find yourself in need of additional functionality.
The best (and easiest) way to fill in the gaps is with plugins. WordPress offers thousands of free plugins via the WordPress Plugin Directory, and many more premium options are available across the web. There is likely a plugin for any element of functionality you can conceive.