The server stream files are the files that contain the custom models, textures, sounds, or other assets that are used by a FiveM server. They are usually stored in a folder called stream inside a resource folder. A resource is a collection of files that can be started, stopped, and restarted at any time by the server. The stream files are loaded by the server and sent to the clients when they join the server, so they can see and use the custom assets.
If someone server dumped a FiveM server(like this guy did) and there is no stream files(like this download OP posted), it means that they did not get the custom assets that the server was using. This could be because the server was not using any custom assets, or because they were hidden or protected by some means. Without the stream files, the server dump would not have the same appearance or functionality as the original server.