Explain in simple language how the communication React to the back end such as Java or PHP?

A REST API. Most often in JSON format.

Contrast oldschool'Noah practice is that the server and the client application are separated.
The second is responsible for visual part of the user experience.The first is responsible for data and business logic. Although it may be different.
The important thing is that the REST API server is not back-end should not be responsible for the generation of html, etc.

Using the http Protocol which runs on top of TCP/IP. It is called REST. Read theoretical books. You're probably one of those who says that why Universities and Theory. Sat down and programmed. But that's What I can recommend is the book of Tanenbaum. Computer architecture. Architecture OS. The third book in a Computer network. Advise the books about Algorithms, Code the hidden language of computer science. Then you won't ask such stupid questions.

Listed pages of books about 4 000. Read at least 1000. selectively. Also pogugliv Wikipedia. This will give you an idea of what is programming. Front End Is an application programming. There must be opoznany in Computer Sciense

If

communication react with buck

means of HTTP requests using AJAX, you can use any convenient technology: window.fetch jQuery.ajax(), superagent, axios, etc.
In simple cases, data loading is performed in the method componentDidMount component.

On the server you have queries Ala api/users and given in the Json data. parse them in the js as you need.
Also on the server. always return the exact same view, and react he understands that to show the client using the internal routing.
There he makes requests to the api where and how to apply, what data it needs.