localhost is (usually set as) an alias for IP 127.0.0.1, which is the standard IP for pointing the local machine.
Thus, accessing the “localhost on another pc” is actually accessing a server on another machine, either through its network name or (network bound) IP.
Accessing a web server from another machine requires that the server machine allows inbound connections.
So, in order to access a distant web server, you need to
check the network name / IP and port used is correct
check the web server allows inbound connections (which may not be the default)
check the server machine allows inbound connections, as @pobrn suggested
If i follow the documentation, React is a pure JavaScript library to embed within HTML pages. This means that one doesn’t actually need to deploy a React application to run it, simply open the page containing it in your web browser.
That said, if you want to have your page accessible outside your machine, you need to deploy your page on a web server.
Alternatively, since it doesn’t actually need a server to run, you can make your page accessible from other systems through network sharing, like any other file: