A plugin is a small add-on that bolts extra features onto your WordPress site - a contact form, an online store, a booking system, backups - without anyone having to build them from scratch.
Also known as: WordPress plugin, add-on, extension
In plain English
WordPress core does the basics; plugins add everything else. Each plugin is a package of code that hooks into WordPress to add a feature - forms, SEO tools, stores, security, page builders. Most WordPress sites run dozens of them.
Why it matters for your business
Plugins are how WordPress does almost everything - and also its biggest source of problems. Every plugin is code you're trusting: outdated or abandoned plugins are the #1 way WordPress sites get hacked, slowed down, or broken by conflicts.
When you'll run into it
Constantly - every feature request ("can we add bookings?") is usually answered with a plugin. The skill is choosing well-maintained ones and keeping them updated.
Run into this on your site?
WordPress Care Plans