If we continue with the “WordPress as a person” metaphor, you can think of plugins like giving your person skills and abilities.
A basic WordPress site will, among other things, allow you to add pages (usually static information) and posts (such as news and other information that needs to change).
A plugin will add additional functionality to your site that doesn’t come straight out of the box. Although they often have to be configured, you can add a calendar of events, a shop to sell physical or virtual goods, and even an entire social network, such as BuddyPress, with a one-click installation.