A plugin is a ready-made add-on that gives your website a new ability without anyone having to build it from scratch. You “switch it on”, and the site can suddenly do something new — show a contact form, an online shop, a photo gallery, a booking calendar and so on.
A simple way to picture it: a plugin is like an app you install on a phone. The phone works on its own, but each app you add gives it a new use; in the same way, each plugin adds a new use to your site. You only turn on the ones you need.
Plugins are one of the main reasons WordPress is so flexible — there are ready-made add-ons for almost anything, so a site can grow feature by feature instead of being rebuilt.