There is actually a lot of material available, including youtube videos. I recommend you have a search for "react isomorphic" and go from there. For the record, "universal" means a similar thing, but also encompasses mobile devices with the use of react-native.
All it means is we render the application HTML on the server and send it straight to the client for client's react to pick up and do less legwork. Faster initial load times and the benefit of having a single code base. This is essentially how we used to do it by rendering our HTML in some PHP backend (wordpress, laravel, zend, sapphire etc) and then using jquery on the client to find some dom nodes and attach interactions to them. Presently this process is more meaningful and less fragmented, so this pattern is back.
Just some examples from first page of results: