It stands for Single Page Application. It's a web application that works by loading a big chunk of JavaScript and using that to render every "page" of the application, rather than providing links to different pages for different parts of the app.

Think Trello (SPA) compared to Hacker News.

These days well written SPAs can use the HTML5 history API to provide proper URLs to different parts of the application, so linking and bookmarks still work.

Historically this hasn't always been the case, and even today poorly written SPAs may fail to implement proper linkable URLs.