I read in AEM Form 6.3 (What's New) that it helps to build Single Page Applications (SPA).
What is an SPA?
How is it different from regular applications?
You would have probably got to known about what exactly is a SPA and how is it different from other regular application. I would now try to answer this question in the context of AEM Forms.
Let's understand this by taking a simple use-case. Say, there is an AEM/Non AEM Page which is already built and you would want to embed/use adaptive form inside this page which is a single page application(meaning no page refresh). Typically in such scenarios you would use the adaptive form inside an HTML container(which could be an iframe or a normal HTML div element) present inside your SPA.
To achieve this use-case, you could leverage the sling based REST API's of AEM Forms to refer the Adaptive Form inside your page. Basically, with the help of XML HTTP request one could bring the Adaptive Form's HTML/JS/CSS from the server to the client.
To go around cross-domain issues, one could leverage/configure the dispatcher/proxy configurations to route the request to the AEM Forms Server.
We are already in the process of documenting this entire feature(using Adaptive Form inside non-AEM page which is a SPA) as part of 6.3, its usage etc. The documentation of using Adaptive Form in an existing AEM page can be found here 
Hope this gives you a brief overview and answer's your query.