The refind-bin file contains rEFInd binaries and filesystem driver binaries
for use on an EFI System Partition (ESP).
The refind-cd file contains a bootable CD image file; burn it to an optical
disc and you should be able to boot rEFInd to test it without installing it
on your hard disk. This image is more likely to work on PCs than on Macs;
at least on some early Intel-based Macs, the CD boot process has special
requirements that this image doesn't meet. This image includes four EFI
filesystem drivers, as well as an open source EFI shell that's not included
in the other packages, which can make it useful for some simple emergency
recovery tasks on an EFI computer.
The refind-src file contains the source code to rEFInd (both the main
program and the EFI filesystem drivers), which you can build yourself. The
build process is based on Linux and uses the GNU-EFI library (for the main
rEFInd program) and the TianoCore toolkit (for the EFI filesystem drivers).
It may be possible to adapt the build process to work under OS X or
Windows, but you're on your own if you try this.