In a traditional multipage website, each URL loads a different HTML page. If you are at the example.com page and need to navigate to example.com/about, the entire view will blink with a page reload because it will need to be rendered again. The problem is that, usually, a page reload is a waste of time since both pages share similar content, such as the page header and footer. Also, CSS and JavaScript dependencies may be exactly the same.
In a Single-Page Application, there is a base HTML file that will be loaded for every URL and, depending on the given URL, the inner content will be dynamically loaded to match the address. Also, the URL browsing is controlled at the client-side using JavaScript. Changing from one URL to another will not cause a full page reload. Instead of loading an entire new file, a request is made to the server to retrieve only what is necessary for the new address and just a part of the page will be rendered again.