VIMRC, SCREENRC, BASHRC, KSHRC, etc. What does the "RC" mean?

There's a bit of interesting historical perspective about its derivation on this page:

"rc" derives from "runcom", from the MIT CTSS system, ca. 1965.

and that

"rc" in Unix is a fossil from that usage.'

It stands for Run Commands

And as the name already says it can contain a listing of commands to run. For example in a xinitrc file You can list commands to run when initializing X. so you can make a Xterm window popup, or load GDM/Gnome.

It stands for Run Commands.