WordPress Plugins extend the features and functionality of a basic WordPress Installation.
Out of the Box, WordPress offers a core set of features which are of general interest to most users, but it (very cleverly) does not try (or pretend) to offer all types of functionality.
Extra “specialist” features are provide with the use of plugins – giving the site owner the opportunity to pick and choose which features and what functionality is most relevant to their specific needs.
There are thousands of “free” plugins available via the WordPress Dashboard, and thousands of other Premium (paid) plugins available online.