Isomorphic apps are the kind of apps that render on both client and server.
The major drawback in Single Page Apps is that they are not very SEO friendly. Google now executes JavaScript while crawling, but you still face problems with other bots. One of the solutions is using a tool like prerender.io to pre render your pages and serve them to bots. Some of the developers also follow DIY techniques and pre-render pages using tools like PhantomJS. But none of these are very nice solutions for a large website.
Isomorphic apps solve these problems. When you request a page, you always get a server rendered version. All the subsequent interactions follow Single Page behaviour i.e. just loading partials. This way when bots request your webpage they always get server rendered HTML instead of binding expressions like {{}}. So, you get best of both the worlds with this approach. Also, many popular websites like AirBnB have already gone isomorphic and it has worked really well.