I think he means if you want to offer React server side you will need some-sort of web framework to serve it. Usually that's Express or Fastify or something like that.
The parent then asked why do you need to integrate [react] with Express and my answer was if you want to do server side (not using Next)
Using Express and Next together is relatively common (or at least used to be): https://github.com/shdnx/next-express