91288384_an_RPM_package___FedoraProject__Action
[Stage] %install [Reads] %_builddir [Writes] %_buildrootdir [Action] 
This reads the files underneath the build directory %_builddir (usually ~/rpmbuild/BUILD/) and writes to a directory underneath the build root directory %_buildrootdir (usually ~/rpmbuild/BUILDROOT). The files that are written are the files that are supposed to be installed when the binary package is installed by an end-user. Beware of the weird terminology: The build root directory is not the same as the build directory. This is often implemented by running "make install".
How to create an RPM package - FedoraProject