A static website is a website with fixed content. It simply serves it's HTML pages and other content as it is. S
On the other hand we have dynamic websites that generates its content by running some server side code each time instead of sending static HTML files. The content depends on the request of user.
WordPress is a tool to create dynamic (also static) websites. It stores all your content including pages post and comments in the database. When a user request a page, WordPress dynamically generates the web page by fetching the content from its database and filling it in the template and send it back to the user.