The support includes the ability to send and receive messages from STOMP clients over WebSocket (incl. SockJS-based fallback options). The application server essentially acts as a simple STOMP broker allowing the application to process messages and subscription requests targeting specific destinations (see #15373).
The other side of this is the ability to plug in a fully featured STOMP broker (e.g. RabbitMQ) with the application server acting as intermediary between connected clients and the message broker. This makes it easy to add application logic as well as to send messages to connected clients from anywhere within the application.
Stay tuned for more details in a blog post on blog.springsource.org soon after the M2 release.