What is HTML formatted data

I have a project i’m working on and i’m allowed to use React and express. But rules are not to use any Html formatted data being exchanged between client and server. What is exactly meant by no Html formatted data. FYI, i plan on using json data to be sent between client and server.

Why does React have a server on its own?

First of all, I know the question is badly formulated. I can’t think of any thing to describe the situation as I’m a beginner.
So when on cmd we type npx create-react-app myapp, this sets up the react project. Then we could do npm start and we could view the react app on localhost:3000. I wish to know why it is so. The react app is on client side, so why does it need a server? Next, I want to set up a node js server and does it need to use the same port as the react app? My guts says no. Briefly, I wish to know why react use localhost:3000 when it is client-side.
Thanks in advance