A Single Page Application is an application that basically operates inside a browser and does not require page reloading during its usage. We are using these type of applications each day. For example, Gmail, Google Maps, Facebook, GitHub are all Single Page Applications.
SPAs are all about serving an outstanding User Experience by trying to imitate a natural environment in the browser with no page reloads, no extra wait time. It is just one web page that we visit and which then loads all other content using JavaScript.