Now I've searched what is the meaning of REST server and REST client but I could not find a good answer. Could someone explain them?
When in doubt, go to the source: Fielding's dissertation where he defines the REST architectural style.
In Chapter 5, he describes Client-Server as the first of the constraints.
Separation of concerns is the principle behind the client-server constraints. By separating the user interface concerns from the data storage concerns, we improve the portability of the user interface across multiple platforms and improve scalability by simplifying the server components. Perhaps most significant to the Web, however, is that the separation allows the components to evolve independently, thus supporting the Internet-scale requirement of multiple organizational domains.
The description of the Client-Server architectural style is in Chapter 3, which Fielding attributes to Gregory Andrews
A client is a triggering process; a server is a reactive process. Clients make requests that trigger reactions from servers. Thus, a client initiates activity at times of its choosing; it often then delays until its request has been serviced. On the other hand, a server waits for requests to be made and then reacts to them. A server is usually a non-terminating process and often provides service to more than one client.