Admob gives you a set of unique IDs which allow it to identify who you are (so it knows who to hopefully pay) and also which app and which platform if you like. You get these when you sign up with admob and create a new ad unit.
They look something like this "ca-app-pub-8312094608055499/3533840015" (this is a real one of mine changed a bit) so the ID is actually a string.
"UNIQUE_ADMOB_APP_CODE" is a sort of metaphor for this code. It doesn't literally mean this
If you have an Admob account (if not get one) goto "Monetize" then click on the "Monetize new app" button, fill in the boxes and it will give you a working code.
