A static website consists on a series of HTML files, each one representing a physical page of a website. So on static sites, each page is a separate HTML file. When you visit the homepage, you are viewing the actual homepage file.
And even if two page contain a chunk of identical content, they both contain two versions. So, if you want to update the footer, you must do so twice, once on each page.
This is fairly straighforward and it's how all websites were built during the eatly years of the world wide web.
And dynamic website uses server technologies to dynamically buikd a webpage right when a user visits a page.
Basically, what happens is the user goes to a certain web address and the server finds a bunch of different pieces of information that it writes into a single cohesive web page, which is what you see.
This way of building web pages "on the fly" has several advantages.