When you first hear about WordPress, it can be hard to understand what it is.
And when you start hearing about themes and plugins it gets even more confusing.
The way I like to explain how it all works together is with a house.
Every website needs a domain and hosting.
The domain is like a name and address in the phone book (if those are even a thing anymore).
Hosting is like a piece of land you buy to build something on.
WordPress is like the foundation for your house. It gives you a strong place to build whatever you want.
A theme is like the shell of the house on top of the foundation.
Just like when you have a foundation and the house, then you can start filling the house with rooms and furniture. The same goes with getting WordPress and a theme you can start filling your site with pages and content.
A plugin is like the extra stuff you put in and around a house.
A dishwasher, refrigerator, water heater, automatic garage door, a pool, security system.. All the stuff that makes it easier and more enjoyable to live in a house.