isomorphic means something like "replacable with something equal" in math. the idea is to replace certain part and get the same output.
for example: object orientated programming and functional programming are isomorphic. you can get the same output from both.
in js they talk about isomorphic rendering where you render the dom on the server side to reduce loading time and bootstrap the framework on the existing dom.
that means in react you have the virtual dom from the server so instead of rendering on the client, you can render on the server so the customer does not get stuck on a 'loading' page