I download the poker-eval library, a open-source resource for poker. I can't install, and get a brief description about the basic functions. So i have a question, how do you retrieve the basic information on an open-source library?
While it varies from program to program, normally there will be files named README and/or INSTALL in the top-level source directory containing instructions on installation and usage. These days most packages include a script, invoked as ./configure, which will detect the relevant characteristics of your system (along with plenty of irrelevant ones) and prepare a makefile so you can run make to compile the program/library, then make install to install it.
In the case of a library like the one you're looking at, there's hopefully also documentation for developers intending to use the library. Look through the README or INSTALL file, a docs directory (if present, etc.). Sometimes the documentation is in the actual source code (in Doxygen format or otherwise) and you can either read it there or use the right tools (e.g. Doxygen) to build externally-readable documentation files from the source.