It is an "Ad Unit ID". From the OpenX documentation:
Ad units are virtual representations of the specific areas in your
site where you want to display ads. You can set up ad units for ads.
When you create an ad unit, you specify attributes for it that will
determine the kinds of ads that will display there. OpenX uses these
ad unit attributes to generate the ad code that will make real-time
requests to the ad server.
You need to set up the ad unit in your admin view of your ad server installation, then copy the assigned ad unit ID to your code in the portraitAdUID
and landscapeAdUID
arguments.