A single page website is one that fits on a single page. Single page websites are fully loaded in the initial page load or page zones are replaced with new page fragments loaded from server on demand, making the experience more continuous and fluid for the user.
Single page websites became a trend recently, mostly among designer sites because this type of web design is perfect for a portfolio. But it can also be found in apps or product sites, basically anything that is not heavy on content. Showcasing all the content of a website in a single page is a great way to save time for visitors as they won't have to jump from one page to another and the site only has to load once.