There are lots of resources on the web explaining WordPress. There are lots of videos on Youtube. Have a look. These beginner videos are free and start at the very beginning. WordPress is software. You install it on a web server. It provides a menu system that allows you to easily create web pages and blog posts. You can easily insert pictures and videos as well. You can create menus. If you want to just give it a try without having to pay anything, go to wordpress.com. There you can set up an account and start using WordPress. You are however limited in what you can do and your URL will end with wordpress.com. If you want to have your own URL, you will need to go to a hosting company and get a domain name and a hosting package.
You can do a lot more as well. The main reason for this is because WordPress extensible with plugins. Plugins add a lot of new functionality, such as ecommerce and advanced Contact Us forms. Many plugins are free and many of them have paid versions that add more functionality to the basic free version.