Like he said, if you've ordered the DVD from RH (RHEL not RHLE), then the 'subscription num' will arrive in the pkg along with the DVD(s).
You only need it when you register the system with RHN. From then on it will remember the num.
# install OS, and it will ask for subscription num.
# You can choose to do this after install;
# if so, then after install, login as root at cmd line and
# run cmd
# and have subscription num ready
The subscription entitles you to (paid) support and updates.
If you are capable of doing your own support, you can get the equiv CENTOS distro, which is a free rebuild of RHEL. You will then be able to get (free) updates from the CENTOS repos, but no (paid) support.