You should be speaking from this page . It is a deses that a group of developers want to make paste. Only time will tell what will happen.
The meaning seems like you already know. It is a technique in which both the server and the client share code. In theory this has advantages. And of course there are disadvantages.
They start from a dangerous assumption that Google can index all rendered pages on the client, which is not a truth absolute and ignores the other search engines.
In addition, it seems to me that putting up not only the rendering, but also the routing on the client, has problems controlling the content. I can not even imagine how you can trust something so important that it's done on the client and has an impact on the server. They should have some solution but should start having its disadvantages.
So it may even be useful for applications, but not for sites . It's no wonder SPA's A is applications and not a S site .
The idea of SPA is interesting and this technique takes it to the extreme leaving very little for the Node.js to resolve on the server side.
It runs into the problem that the current technology does not work so well with SPA and many applications fail to get along with the extremism of letting the server just deal as an API provider and nothing else.
So it seems that few have bought the idea so far.
The obvious advantage is the purism of using only one language, a single coherent library. Besides not overloading the server. In theory.