Repo=software repository, where the rpms are stored on RH's (and 3rd party) servers. IF you are going to use RHEL you need a base repo(where the basic system stuff is stored for yum to get stuff), essentially in order to run RHEL you need to have a paid RH subscription. The third party repos only have rpms that are NOT in the base repo, so you need a base repo to use them.
If you do not have or cannot afford the RHEL subscription, you should consider switching to Centos. Centos is RHEL with the logos removed. It is free to download/update (unlike RHEL) and is binary compatible with RHEL.