Signal-R is a library component that brings out real-time web functionality in the application. Real-time basically means the ability to have your server-side code push content to the connected clients as it happens, in real-time. SignalR uses the new WebSocket transport.
What can be used for?
Pushing data from the server to the client (not just browser clients) has always been a tough problem. SignalR makes it easy and handles all the lifting for you.