, probably "runtime configuration" or "resource configuration", just a guess. It's not only used for the init system configuration stuff, but also, under your $HOME there will probably be some dot files that end in "rc", like ~/.bashrc and so on.
stands for "ram disk". The initrd image is a file that contains a small filesystem that is loaded into ram when booting, and usually contains drivers are are needed before the kernel can access the disk at a file system level (for example, the file system drivers, which can't be loaded from a file system that's not working yet, because well, the drivers have not been loaded
you get the idea).