Single Page Application
A Single Page Application is a type of application where only one page is loaded from the server.
What does that mean?
Usually when a user types a URL on the browser or clicks a link, a request is made to the server. The server processes the request and renders a page back to the browser. If a user clicks on a button or link on the page, the server processes the request and renders a brand new page. However in a single page application the server does not render the complete page again it just reloads parts of the page which needs to be re-rendered. If you view the page source on the browser you will only see the same html with a bunch of javacript tags.
Below is a good YouTube Video briefly explaining Single Page and Multi Page Applications: