You put on your socks, but then you realise that one is black and the other is white. Do you take them both off and put on two socks of the same color or do you keep one and substitute the other with a matching sock? A Single Page Application works using the logic of the second case.
An SPA is a web app that presents user interface with optimised content loading. Rather than search for all of the content each time you surf the pages, SPA keeps a trace of the content that has already been used and keeps it available. For example, menus, navigation bars and other elements are not refreshed with the variation of the pages. Gmail, Google Maps and Facebook are all SPAs, but there are many others.
SPAs guarantee fluid navigation and their perceived speed is higher compared to all other Internet sites. These aspects mean that the technology has a high impact on user experience.