a file that has .conf at the end of it is a configuration file. These are text files that you can edit to get something working. the yum.conf is a file that is located in the /etc directory, you can edit it using your favourite editor - I like jed, but you can replace jed with emacs, vi, vim, pico etc- whatever you have and like.
# jed /etc/yum.conf
and then follow tkedwards' first step- which means type (or copy) that text into the yum.conf file, then save, exit and follow his second step.